A square jaw, a protuberant chin. A manly cleft. Somewhere along the line, artists decided that these, more so than anything else, were the defining facial feature of really heroic (or strong and dumb) characters. Perhaps to an even greater extent than broad shoulders, barrel chests, and absurdly large biceps, the surest sign of a character's strength of character and manliness lies in his chin.
Superhero parodies will often take this to an illogical extreme, giving the hero a jawline large enough to keep any normal person from being able to look down.
Often a characteristic of a Top Heavy Guy.