I agree, Dwarf frogs are small, and you should be rather concern of a fish eating your frog, not a frog eating your fish.
Unfortunately, these frogs are very often mislabeled in stores.
What you should remember is
1. that any dwarf frog that is labeled as albino - is NOT dwarf, but Clawed - Clawed frogs grow bigger than your fist, and can eat fish equal their size (I actually saw a guy feeding them with small mice).
2. I think the easiest way to distinguish them is to look at the position of their eyes. Clawed frog has eyes placed on the top of its head, Dwarfs have eyes on the sides.