Technically, belly, a slang term for stomach, is itself a muscle. It can contract and expand, usually based on the volume of food ingested. Those whose stomach work properly have good muscle bellies, those who have indigestion have bad muscle bellies. Now whether or not its full is individual. Currently, my belly is full, as I just finished a meal. About an hour or so from now, though, my belly will revert to not being full.