To be honest, there isn't much difference between these two animals. Concerning husbandry, Leopard geckos have been known to eat their substrate. So they should be kept on reptile carpet or paper towel. Fat tails on the other hand, are less prone to eat substrate and can be safely kept on Eco Earth, or coconut fibers.

That's really the only difference I can think of. I'm not sure if fat tails are bigger, but they do seem to be "stockier". They have relatively shorter limbs and digits compared to leopard geckos and their abdomens are a little rounder.