Big swords are mostly for aesthetic appeal. And why not? Each writer has in their ability to create their own, fantastically messed up little worlds. Why not create a world where it makes perfect sense for a normal human to swing around a huge sword? That's like calling urban fantasy unrealistic because, after all, magic doesn't really exist!!
Both the Scottish claymore and the great swords used by the Irish Galloglass warriors tended to be about a foot shorter than the men who would swing them. I saw a reference to the "Wallace Sword" that's being sold in catalogs a lot in the past decade. The genuine article - an original blade used by Wallace but re-hilted sometime in the 16th century - is 66 inches long, with 55 of those inches being blade. Wallace himself was over six and a half feet tall, at a time when the average man was about five-foot-six. Since most of Wallace's career was spent ambushing English patrols and convoys with small guerrilla forces, I'm betting he used an effective weapon.
These swords were brutally effective against pikes, mounted cavalry, and poor slobs with lighter weapons - think about it. This big guy with a sword twice as long and twice as heavy as yours comes at you. What do you do? These swords were long, but also fairly light, and the men using them were freakin' huge. The intimidation value on the battlefield would've been immense. The power in a swing would've been similarly immense. The real trick was balancing the size of the weapon against the weight of the weapon.
Now, a lot of "experts" will tell you that Claymores couldn't be used effectively because they were "too big", or some such. Bear this in mind:
The claymore was in use in Scotland from before William Wallace's time (He was active around the year 1300) until its last recorded use in battle, the Battle of Killiecrankie in 1689. That's almost 400 years, and you know that Wallace didn't invent the thing so it had to have a history before him. A weapon doesn't stay in use that long unless it works and works well.
