Much weight on a frog is the water which they absorb while soaking. It is probable that he is eating when your not looking and if your finding droppings from both frogs then you know he is eating something.

He may not be going after the feeders right off due to the other frog being more aggressive towards getting to the food. I would monitor his weight, ensure your finding droppings from the frog and keep up what your doing since he looks fine in the photo.