Okay, I'm going to take a stab at some xenopsychology (a valid term here, since Japanese culture is alien to me) here. A lot of characters (especially main characters) in Japanese games, anime, etc. have physical power completely out of proportion to their actual physique. This is because these stories go in big for the character having some unusual quality, like "great spiritual strength", "exposed to some super-soldier process" or "training beyond the ken of ordinary people". One of the easiest ways to express this hidden strength is to have the character wield some ridiculously huge weapon- especially if the character is a slender, large-busted woman whose arms barely seem capable of holding up her hands, let alone a 60-pound chunk of sharpened metal. Seeing this svelte character swing a weapon that puts claymores and zanbatou to shame, and use it in a way that would have any Olympic fencer or kendo practicioner green with envy, shows that the character is far above his or her more mortal peers in power.
Or it could just be that Rule of Cool thing, yeah.