From what I understood and my personal experience with it, your pet will need a lot of food at first to maintain it happy as long as its loyaltly level is low. Progressively its loyalty level will improve and the more it does, the less food your pet requires.
Another thing to keep in mind is that when you feed your pet you need to keep it out of combat for the duration of the feeding buff (20 seconds I think). If you don't, your pet won't get the full benefit from the feeding.
Good luck !