Platy fry question

About a month ago, I split up my Platy fry born the last week of January, 10 to the big tank, 10 remain in the nursery tank. I find it odd, that the 10 in the big tank, seem to be the same size as they were when I put them in a month ago. The 10 that remained in the nursery have nearly doubled in size in the last month. Why the growth difference between the two groups of fry, born on the same day, yet in different tanks?

Does what tank get more water changes than the other?
could it also be the speed of the water moving?

The big tank gets 30 to 50 percent water changes twice a week. Nursery gets 30 to 50 percent once a week. I have a Marineland Penguin 350 in the 25 gallon tank, and an AquaTech 20-40 in the nursery.

Could it be the ability to find food? In a smaller tank the same amount of food is more concentrated and so easier to find. Also if their are other fish involved in the big tank they could be eating a portion of the platy's food and so relatively they aren't getting as much as those in the smaller tank.

Sam