A stomach that is noticeably large, as if decisively full, fat, or pregnant. Commonly used when depicting a character that has eaten a great deal of food. Not necessary when depicting pregnancy unless the character's belly is very large even in spite of their current condition. Can be used in indecisive cases where it's not clear if the character is actually pregnant.
For a midriff that is only somewhat plump or flabby, use belly instead. Avoid using both belly and big belly, as they generally do not overlap.
