I think if he seems to be strved after eating his food he may have tape-worm?
I studied these worms in school and they can start out quite small and can just keep on growing. One of my neighbors cats had one and she didnt start showing that she was underweight for quite a while after she got the darn thing
And how much your feeding a dog depends on its energy level,and other things. You may have 2 of the exact same breeds from the same litter, but one will eat very little and stay healthy while the other may eat like a horse and stay at a healthy weight. Its all a matter of metabolism and whatnot. Feeding instuructions are just guidelines. It differs from dog to dog.
Good luck! I think you should consult a vet and rule out any health preoblems( i.e. worms)
BTW- Welcome to Pets.ca!