I've been shrinking out of my kilts for a while. I had a casual wool kilt that I decided I could put an extra pair of holes into the straps to snug it up a bit. It worked OK, but keep in mind that there's a limit to the amount of leather you have left. My buckles on that kilt are snugged up right onto the tartan, and it looks a little weird when I don't wear a belt.

If you need to take it in more, the only alteration you will need is to move the inside strap and the outside buckle an equal distance toward the center of the kilt. This is a minor alteration, and if you talk nice to the right person, you might even get that done at no cost.

Regards,
Rex.