Many dogs don't have an "on-off" switch when it comes to appetite (Labradors spring to mind!). These dogs will act as if they are starving hungry, even though they've just eaten. If your dog has always been greedy, then it's likely that's just part of who they are. To be on the safe side, check the dog isn't under weight (a vet tech will happily weigh the dog and advise you), and deworm them.