How do you make both a weapon and its wielder more impressive without making them overly complex? Well, Bigger Is Better, so making the weapon dwarf its wielder makes the weapon more formidable, and shows off the wielder's strength, usually making them a Pint-Sized Powerhouse, having them use something that would be otherwise impractically heavy.

Not all BFS, BFG, and the many other great-weapon trope examples would count as this, because sometimes those are just bigger versions of regular weapons, but attached to a similarly big person to match.