If you are talking of more than just an inch or two, making the waist larger will all but destroy the integrity of the kilt. The only way I could see to do it would be to completely disassemble the entire kilt and re-pleat it, making the pleats larger. Even then it is questionable if you could do it, as the excess material from each pleat is trimmed off at the waist during construction.

Any others have suggestions out there?