Certainly if the kilt no longer fits correctly that would contribute to the discomfort factor. A properly fitting kilt should be comfortable because it is made to your body size and shape.

However, if you are looking for a more comfortable, lighter weight kilt, I would suggest keeping the heavy weight wool and opting for a lower yardage kilt, as opposed to an 8 yard kilt made from light weight wool. This way you keep all the advantages of good heavy weight tartan, but still have a lighter kilt.

The most comfortable kilts I have worn have been 16 oz heavy weight, four yard box pleated kilts. They weigh about half as much as your typical eight yard kilt, and have approximately the same amount of cloth in the front and in the back, so they balance very well on your body. You hardly feel like you have anything on!