For me at least, larger knives are a lot easier to grind and get to balance. The taller in height the blade is, the easier it is to keep even on the grinder, it's the short (in height) blades that are a pain in the arse. With less contact area against the belt, its harder to "feel" your grind and a lot easier to screw it up with a simple turn of the wrist. As far as balance goes, its easier to balance out a knife when there's a good amount of blade out front to counter the handle. On a small knife, granted that balance isn't as important, but it's tough to move the balance point forward when there isn't much weight in the blade. Maybe it's just me, but smaller knives are more difficult in a lot of areas.
So I don't think the issue is difficulty, I think it's usefulness. My flaw is that I like to make knives that I would want to use, and I find that I never carry a large knife. I think you can do most everything with a blade under four inches.
Having said that, one of the blades i'm working on right now has a 6 1/2" ege and is just over 12" overall. I really like the design, it just seems to flow and have a good proportion about it and I think its going to be a really gorgeous knife......but I don't know what the hell I'd do with it. It would be a great "camp" knife, but I've never needed more than a 3" blade and an axe. In the end, maybe it doesn't need to do have a purpose and it can just be a large knife.
I do love the look of the large fighting knives, there's a thread in Customs and Handmade called "Lets See Some Pure Fighters" that is just pages and pages of the most beautiful knives I've ever seen, but the truth is, I'd never carry a big knife like that.
I'm not opposed to the idea of making larger knives though, just need some convincing.