These have grown out for me just like any other fish. I have 2 that are 6-7 inches now. Ime picking fish free of deformities (beaked face, oddly sunken bellies, uneven jaw) helps your chances a lot.
The price of these fish are so high that the people breeding them rarely cull. Every living fish, healthy or not, makes it to the fish store to maximize profits. Really gotta prick your fish carefully