You just gotta' play it by ear.
"Cats 101" presented a story about an insanely huge cat (i forget the breed). Nothing seemed wrong other than his size. After nearly all avenues had been exhausted, the vet asked how much were they feeding the animal. It was no more than what was suggested by the package. He said try cutting it in half. Guess what? The cat started losing weight, but not so fast as to cause other medical issues. So they continued to follow that regiment.
So, at least in some cases, not all, but some, those package estimates are excessive. If your cat doesn't want to eat that much, but is otherwise healthy, I wouldn't force it upon them. But let your vet be the proper judge on these matters. I have no training or pets, so what do i know? ;-)