I agree, even with a kilt, the tie (if not hidden by a waistcoat) should end at the top of the kilt. This makes it much shorter than when worn with trousers, and will indeed make the knot thicker since the wider part of the tie is used in the knot. But that's the way it seems to be done in THCD.

I always try to get it there, even with super-fat ties like this one. And to be honest, I actually think a nice fat tie (with a fat knot) works pretty well with Highland dress.