In medieval warfare, the increasing effectiveness of armor naturally prompted countermeasures, among them the war hammer, which could either crush armor with blunt force or punch through it with a hard spike on the reverse side. In fictional depictions of medieval warfare, though, the heads on war hammers have grown to incredibly large sizes: they're often larger than the wielder's own head or more, sometimes even larger than their entire body. This trope can also apply to maces with gigantic heads, but maces are somewhat less common as a fictional character's preferred weapons than war hammers.