I actually thought that Duncan's kilt looked more like a lighterweight casual kilt. I guess it depends on what series you are watching. Apparently one kilt was used for series 1 and 2, a new one was made for series 3, and than another for series 4 (not sure after that). So whle it may look more like a casual kilt (less pleats and more bounce) in the later series, maybe it was heavier in the early shows. I always thought that Hector, Archie, Kilwhille, and Golly (in 1 and 2) had heavier looking kilts.

It's interesting to note that Golly went for the MacDonald kilt (At least I think it is) ]in series 1-3

But wears a different tartan (not sure what but it almost looks like Weathered MacKenzie) in the later seasons . That kilt too looks like it is more of a casual kilt than the previous seasons.