Baby Grove Snails Growing Slow

[Bogs]

Almost a year ago my grove snail, Nemo, laid a bunch of clutches of eggs, and they hatched and eventually I made a nursery tank, and since then they've grown, narrowed down to about two dozen young with the largest ones being up to an inch or so long, with shells around 1/3 of an inch. However one hatchling was a straggler in the adult tank, Slargo, who grew to full size in just a few months. Are the other ones slow growing because of food shortage/competition? Is their enclosure too small? They all have proper calcium sources. Is it at least somewhat normal, and can they recover and grow to full size after being stunted like this?

Any help is very appreciated.