Hi, Julie... peace to you!
There is something with regard to this matter that many miss:
Adam was taken from OUTSIDE the Garden, and placed INSIDE the Garden. That garden, however, was NOT physical... but spiritual. It is the Garden of the City of New Jerusalem, which is not a physical place, but a spiritual one... the kingdom of God.
Now, you are correct, on the PHYSICAL earth, such would be impossible: populations... of man AND beast... would increase to the point of overload. The physical earth, however, was that which was OUTSIDE the Garden. When Adam was placed IN the Garden, however, he was NOT placed there in the physical flesh, for the physical flesh... flesh "with its blood"... cannot 'enter' into the kingdom. So, he could not just simply walk into the Garden, intially. God... had to place him there.
In order to do that, Adam had to be given the kind of flesh that COULD enter... the type of flesh that my Lord had... that which could 'transfigure'... from the physical to the spiritual. Could 'go in and out'. Like spirit beings... angels, etc. That being the case, a body that could 'transfigure' into spirit, there is NO set number of that which can reside in the kingdom, because unlike PHYSICAL matter... which takes up SPACE... that which is spiritual is not subject to space... or time.
You see, in the physical world, no two pieces of matter, no matter HOW small (thus, including atoms and their parts), can occupy the same space. If one atom is here, another cannot be. In the spirit realm, however, such 'laws' are nonexistent. That is the reason WHY that realm... "Jerusalem Above"... is FREE! It is not BOUND by the 'laws' of the physical realm.
Once Adam 'sinned', however, he was given a 'long garment of skin' (NOT an animal skin, mind you, for such an animal would have had to die, yes?) that 'enslaved' him... BOUND him. This 'garment' was the physical body that WE possess now... that which we 'inherited' from Adam. It is the 'befouled' and unclean 'garment', called such because it holds WITHIN it... sin and death, in the form of sickness and aging. The spirit body does not age nor does it carry illness. It is not 'corruptible'. We tend to think of 'corruption' as crime, etc. But in truth, 'corruption' is simply decay. From the time we are born, our phyiscal bodies begin to decay... until it returns to the dust.
The spirit body, however, is 'incorruptible', in that it is immortal. Now, some tend to think of immortality has not needing food, and not dying. The first is an error: even spirit creatures must eat. However, the second is true... almost. While the spirit body does NOT die (thus, it is 'immortal')... it CAN be destroyed. In fact, it MUST be, to be terminated. The only thing that CAN destroy it, though... is fire. Water can contain the spirit, but only fire... can destroy it.
To reside IN the Garden, Adam had to 'put on incorruption'... or a spirit body. This was given him by God (a 'white (thus, uncorrupted) robe')... and such body was SUSTAINED... by means of eating from the Tree of Life. When he was cast out, however, the very purpose of the cherubs posted was to keep him from getting BACK to such Tree... and eating... which he could do to 'undo' the 'decay' that was now occurring; REVERSE the death process and live forever. Because as long as he ate from that Tree... he would live. The decay process... would be reversed... and he would be 'restored'... to life.
The reason for this confusion is that many miss the point that the account in Genesis talks about TWO 'earths'... that which is OUTSIDE the Garden... and that which is INSIDE. That which is physical... and that which is spiritual. Outside the kingdom... and inside. Outside the kingdom, you are correct: the earth cannot sustain such populations effectively. Inside, however, there is no such problem.
People tend to think of the kingdom of God as being some light years away ("somewhere in a galaxy far, far away...") that one can reach simply by traveling some distance physically. However, as my Lord said... "the kingdom... is in your midst." God, then, is NOT 'far off' from anyone of us, and is, in fact, within groping distance. All we need do is reach for Him. We can't reach, however, with the physical flesh... for such flesh "is of no use at all."
Such 'groping', then, must be done... WITH THE SPIRIT. For God IS a Spirit... and those worshipping Him must WORSHIP Him, approach Him, seek Him, speak to Him, hear Him... and SEE Him... with spirit.
I truly hope this helps.
I am your servant, and a slave of Christ,