Krsna is the source of all Narayana/Visnu forms of God who are further expansions from Krsna's first expansion Balarama. However, Krsna is always the original Supreme Personality of Godhead and cause of ALL causes.

The original and similtaneously infinite Krsna eternally resides in His personal abode known as Goloka-Vrindavan and NEVER leaves.

Srila Prabhupada - "Kṛṣṇa as His original childhood form NEVER leaves Vṛndāvana, all the forms of Kṛṣṇa that appear elsewhere are His Viṣṇu-tattva expansions." (BG, Ch 10 text 37)

Srila Prabhupada - "The original Lord Kṛṣṇa never leaves Goloka Vṛndāvana. All the plenary expansions are one and the same Viṣṇu-tattva, and there is no difference in Their potency." (SB, Canto 3 Ch 1 Text 34)

Srila Prabhupada - "Kṛṣṇa is the original Supreme Personality of Godhead, and Baladeva (Balarama) is Kṛṣṇa's immediate expansion. Because Kṛṣṇa NEVER leaves Vṛndāvana, all the forms of Kṛṣṇa that appear elsewhere are His expansions." (BG, Ch 10 Text 37)

However, when Krsna wants to enter the material world, He personally does not come as His original form but instead sends His first expansion Balarama, who plays the role of Krsna in the material creation as explained above.

When Krsna seemingly enters the material world, it is actually from His first expansion Balarama that all the unlimited Visnu-tattva expansions expand from, including the Visnu-tattva expansion who plays the role of Krsna in a facsimile version of Goloka-Vrindavana in the material world. 

Two major difference between the Goloka-Vrindavana in the spiritual world and its facsimile in the material world, is Krsna never kills demons in the spiritual world, and the gopis and other inhabitants of Vṛndāvana never feel separation from Kṛṣṇa in the spiritual world, that only happens in the Goloka-Vrindavana facsimile in the material world.

Krsna through Balarama, further expands Himself as a Visnu who plays the role of Krsna outside the spiritual world in the material world.  

In the spiritual world (Goloka-Vrindavana), Balarama is the older brother of Krsna which is a paradox because Krsna is the Supreme of Godhead and the cause of ALL causes.  

Krsna is the first therefore the original and for infinity is the source of all the unlimited Narayana/Visnu forms of God, each with their own name that gives insight into their particular pastime.

The Visnu/Narayana expansions reside in an unlimited number of perpetual Vaikuntha planets that surround Krsna's central abode of Goloka-Vrindavana all together taking up 75% of the Spiritual Sky)

Krsna further expands, via His first Visnu-tattva expansion Balarama as Maha-Visnu (Karanodakshayi Visnu) who creates and manages the unlimited temporary material universes called Brahmandas that take up 25% of the Spiritual Sky.

These unlimited number of Brahmandas are coming from the Body of Maha-Visnu through His breathing and pores.

Each Brahmanda universe is massive in size and each houses a "secondary smaller" spherical material universe inside of them.

Each individual material universe’s outer Brahmanda covering is made up of material elements (both subtle and gross)

Each massive Brahmanda material universe covers, encases and surrounds the inner material universe that is deep inside them with 7 layers of material elements.

Maha-Visnu expands again as Garbhodakashayi Visnu who creates each secondary material universal sphere deep inside each Brahmanda.

Then from Garbhodakashayi Visnu, Brahma manifests who creates the different planetary systems within each inner secondary material universe.

There is only one Maha-Visnu however, there are unlimited Garbhodakashayi Visnus and Brahmas who are in their particular secondary material universe.

Due to the massive material elements that make up the outer Brahmanda universe, no one inside their inner material secondary universe can see outside of their own universe Srimad Bhagavatam explains.

This means from within our small secondary material universe inside our Brahmanda, we cannot see the billions of other Brahmandas around us.

Also each secondary material universe deep inside their Brahmanda, has a different amount of planetary systems according to Srimad Bhagavatam basedon how many heads their Lord Brahma has.

In other words, the Brahmanda universe and its inner secondary material universe inside it, all come in different sizes because some Brahmas have 4 heads, 20 heads, 50 heads, 1000 heads, even 100,000 heads, all inconceivable to the non-devotees.

Karanodakshayi Visnu (Maha-Visnu) is sleeping and DREAMING the activities within each of the secondary material universes that are deep within each of the unlimited number of Brahmanda universes.

Those dreams Maha-Visnu has, are all the activities happening within each Garbhodakashayi Visnu's secondary universes.

In painting below Karanodakshayi Visnu (Maha-Visnu) is seen laying down sleeping and Garbhodakashayi Visnu is seen within each of His Brahmanda universes.

When Maha-Visnu dreams, He first creates the entire material universe known as the Maha-tattva (matter) which takes up 25% of the Spiritual Sky or Brahmajyoti,

As said above the other 75% is the active perpetual spiritual world (anti-matter) that is ever fresh, always youthful and never ending, known as the eternal Vaikuntha planets of Visnu and Krsna's central abode of Goloka-Vrindavan.

Within Maha-Visnu's 25% of the Spiritual Sky called the temporary material world (mahat-tattva), we find it is always in a state of decay, decline, impermanence, birth, disease, old age and death requiring constant maintenance.

Each eternal individual jiva-soul who chooses to enter the material creation, are forced to experience repeated birth and death by possessing a material bodily vessel, a temporary material body or container that temporarily houses the individual jiva-soul.

The visiting eternal individual jiva-souls in the material creation take repeated births again and again and again because each material body they are in is temporary and subject to getting old, wearing out, breaking down and eventually ceasing to operate.

The conclusion is-

1 - Within this temporary material creation we find unlimited individual massive single Brahmanda universes coming from the Body of Maha-Visnu.

2 - And deep inside each of those massive Brahmanda universes, there is a "secondary smaller material universe" where Maha-Visnu expands Himself as Garbhodakashayi Visnu in each one.

3 - And from Garbhodakashayi Visnu, Lord Brahma appears from His naval at the top of a lotus flower in each secondary material universe.

4 - Then Lord Brahma builds the many planetary systems inside each of the inner secondary universes deep inside their individual Brahmanda that can be 14 planetary systems like in our secondary material universe (our Brahma has 4 heads) or many, many more planetary systems depending on how many heads their Lord Brahma has.

As explained above, the number of planetary systems deep inside each Brahmanda's "secondary material universe", vary in size, some larger and some smaller than others.

Our material universe within our Brahmanda has 14 planetary systems in it, and is considered a small material universe explained in Srimad Bhagavatam as being only 4 billion miles in diameter.

Some Vedic Cosmologists think this figure of 4 billion miles in diameter describing our material universe inside our Brahmanda, is way to small of a figure.

Many scholars suggest the figure of 4 billion miles in diameter (500 million yojanas), as described in the 5th Canto of Srimad Bhagavatam, is really a code that needs decoding because they claim just our solar system alone is said to be 22 billion miles in diameter, and that the planet Pluto is 5 billion miles from our earth planet, recently visited by a NASA space-craft.

Another answer is the measurement of ''space and time'' is different on each planetary system, just like the fabric of "space-time" moves at a different rate in each of the 14 planetary systems.

For example, 100 earth years on Brahmaloka, the highest planetary system out of the 14 systems, is 311 trillion and 40 billion human years long experienced on our Earth planet.

As also said above, other "secondary material universes" inside their Brahmanda, have many, many more planetary systems in them than just 14, and are obviously much bigger than our mere 4 billion miles in diameter.

The fact is, as explained above the size of each Brahmanda and their inner secondary material universe, depends on the how many heads Lord Brahma has, some Lord Brahmas, like ours, has only 4 heads, while other Brahma's within their universe inside their Brahmanda, may have 10, 20, 50, 500, 1000, 100,000 or even a million or more heads.

In this way, all the quadrillions of Brahmanda universes coming from the Body of the sleeping Maha-Visnu (as seen in painting below), each have a "secondary inner material universe" deep inside them as seen in painting below.

Each massive Brahmanda outer universe has 7 material coverings or layers that surround and enclose each of the inner secondary material universe, this means the massive outer Brahmanda surrounding universe we are deep within, is 44 quadrillion 444 trillion 444 billion miles in diameter.

This explanation maybe the full size of the Brahmanda or could be just the size from the outer edge of the inner secondary material universe.

If that is true then the full size of our Brahmanda is double the figure mentioned above.

In the Srimad Bhagavatam Bhu-Mandala is described as the “earth mandala” (not to be confused with our small earth globe), is a disk 500 million yojanas in diameter (4 billion miles in diameter)

The word ''earth'' has many meanings.

For example, our small earth global round sphere is only 24,901 miles in circumference and is 7,917.5 miles in diameter, it is NOT Bhu-Mandala.

Our "secondary material universe" is 500 million yojanas (4 billion miles) in diameter or 12.6 billion miles in circumference.

A yojana is a unit of distance about 8 miles long. 

So the diameter of our "secondary material universe" is about 4 billion miles in diameter.

Our earth planet is NOT flat! It is a round global sphere.

Srila Prabhupada - "On the ground our earth looks flat but when you go up you will see our earth planet is a ball." (Lecture New York City 1966)

The Sanskrit word "dvīpa" has two meanings

1 - "Islands" or "continents" on a planet

2 - "Globes or spheres" in outer space.

Bhu-Mandala is marked by circular features designated as islands (dvipas that can also mean planets) and oceans.

Srila Prabhupada -"The "planets" are called "dvīpas". Outer space is like an ocean of air. Just as there are islands in the watery ocean, these planets in the ocean of space are called dvīpas, or islands in outer space." (CC Madhya 20.218, Purport)

Therefore, Bhu-Mandala (which is really the 4 billion diameter secondary material universe) is far too big to be our small earth globe. 

However, this does describe our small single secondary material universe with it's 14 planetary systems that are deep inside our surrounding Brahmanda.

Therefore, the 14 planetary systems deep within our surrounding Brahmanda, would fit in the circumference of Bhu-Mandala (4 billion miles in diameter, which is 500,000,000 yojanas in diameter)

The individual jiva-souls are eternal, they have no beginning point, nor will their existence ever end. Therefore, if they choose, they can stay in the temporary decaying material universe's cycles of annihilation and creations, for almost an infinity! 

Or make the right choice and return back home, back to Godhead (Goloka-Vrindavana where there is no birth, disease, old age and death.

When Lord Brahma dies after 100 of his years (311 trillion and 40 billion human years), there is an equal period of his life span of 311 trillion, 40 billion human years where nothing living exists, a gap in space-time where everything in the material universe remains dormant for that enormous amount of time.

Then the next material creation of Brahmanda's manifest from Maha-Visnu (Karanadakashayi Visnu)

All the eternal individual jiva-souls have merged as "individual sparks" temporarily into the Body of Karanadakashayi Visnu (Maha-Visnu) and remain that way until the next creation of Brahmandas appear when they are again created from Maha-Visnu's dreaming spiritual Body as seen in painting below.

In other words, in the next new creation of Brahmanda universes coming from the Body of Maha-Visnu, all the dormant eternal individual jiva-souls awaken and continue on again within a "secondary material universe" that is deep inside each Brahmanda from where they left off in the previous inner secondary material universe.

As explained above, each secondary material universe is deep within each of the massive surrounding Brahmanda outer universes that all lasts for 311 trillion and 40 billion human years, or the life span of Lord Brahma.

It is important to understand at the annihilation of each Brahmanda universe, and its inner secondary universe, Garbhodakashayi Visnu temporarily merges back as one with Maha-Visnu until the next Brahmanda universes manifests from the breathing and pores of Maha-Visnu.

Then the process repeats itself, the sleeping Maha-Visnu (Karanadakashayi Visnu) expands Himself into the inner secondary universe that is deep inside each Brahmanda as Garbhodakashayi Visnu that is in the Centre of each Brahmanda universe.

The eternal individual jiva-souls are suspended in ''inactivity'' (dormant) within the Body of Karanadakashayi Visnu (Maha-Visnu) for 311 trillion 40 billion human years.

The jiva-souls re-emerge and continue on (their karma) from where they left off in the previous creation.*××*