There is also the feeling in some aquaristic circles that larger and adult fish may actually release a "growth inhibiting substance" which keeps the smaller fish, smaller. While I'm sure it is possible, all cases of that are not necessarily releases of a hormone.

As mentioned in too many of my comments here (obviously a guilt factor for me too) too high a level of fish waste in the water can lead to at least partial nitrogen poisoning. That is where the nitrogen (ammonia, nitrates or nitrites too) level rises in the water and, via the fish's gills, enters into the blood stream. The nitrogen can actually block effective the carrying of oxygen by the hemoglobin and the fish are unable to as effectively metabolize their food for growth and other body processes. That means a greater percentage of their food passes through their systems and - you guessed it - dumps more nitrogenous waste into the water. :(

That is why urban mothers may not wish to use tap water for their baby's formula. The ammonia in the chloramines can endanger human infants too.

( Back to the guppies. I hear Dr. Charles in the recesses of my mind, chanting,) "Change more water". And that is one of my next projects here too. ;)

In an ideal world, you also might be able to split up that tank into a tank of large fry and a tank of smaller fry. Oh yes, and provide more filtration, maybe using activated carbon. ;)

Not so much with livebearers as some other fish, but people raising fry will also sort their fry by size, because if they don't, they will have fewer  females (as with some killies and cichlids) owing to cannibalism and bullying. With some livebearers and other fish where males are smaller, that may mean that there are fewer males available. Usually with over motivated livebearing males, that is not a problem. However there is a subfamily of the Poeciliids where the males are a lot smaller than females and at risk around females. I forget the precise name for them - Gambusids? Gambusini? - (and don't have John Dawes' book on livebearers handy.) But people raising the livebearers related to the Gambusias (including not only the Gambusias, but also the very cannibalistic Pike Livebearers and the combative, but pretty Central American genus Brachyrhaphis, may find to their dismay, that out of a too small, shelter-less tank that they have NO males from that batch of fry!