If the difference in size is less than 2 inches, it should be pretty easy to adjust the straps. You'll even have a better looking kilt, as the apron will be closer to half of the circumference!

Someone has posted detailed instructions, but I don't find them immediately. Here's the short version.

On the right side, move the buckle farther to the right. I'd set the waist strap properly before removing the hip strap: you may not need to move it; or you may want to remove it altogether.

On the left side, move the strap on the under-apron, also to the right. The under-apron edge will overlap past the buttonhole---that's fine. You may want to add a bit of fabric behind the apron, as reinforcement; since it will no longer be attached at the hem.