I remember when I used to feed my Pacu until she was all round and potbellied, and I could see her moving around lethargically and taking much bigger, deeper "breaths", which is pretty much what people who overeat do, right? That can't be good.
Fish do "bulge" eat every now and then in the wild, but it could be weeks before they get such huge meals. How do I know? If there was enough food for fish to bulge themselves every day there would be catastrophic overshoots and diebacks in their populations (availability of food would cause their populations to explode, but then other limiting factors would push them back down). However, their populations are for the most part stable, meaning their is an equilibrium between predator and prey. Predators stuffing themselves until they look like bowling balls is usually not considered equilibrium.
I think that while nutritionally complete meals are important, it's not necessary to stuff our fish like turkeys with these foods. A slight concave portrusion in their sides is enough for me to stop feeding.