I've bred numerous different types of fish, and it's my experience that dwarves stay small. This is why the best breeders are mercilous in their culling. I know a lot of Tilapia breeders keep their fish separated by size (commercial AP systems seem to have 3 or 4 FTs for different sized fish), and maybe some fish stay in smaller ponds indefinitely. There it doesn't really matter b/c they're still adding nutrients for the crops. In a backyard system that has limited space for FTs, it might be best practice to cull smaller fish, allowing more food to go to those fish that are growing quicker. If you know your population won't be supported once all fish reach adulthood, you may want to cull the runts now.

Food for thought...