2 inches is about the maximum amount you can take in a tank without throwing off the look of the kilt. But it can be done quite simply.

The first step is to put on the kilt without putting the straps through the buckles. Insure the kilt is on snug but not tight and that the waistband is aligned along the top. Have a look in a mirror to check that the kilt is hanging correctly, the pleats are not splaying and the apron not puckering.

Mark the kilt on the buckle side with a piece of sharpened blackboard chalk along the edge of the outer apron from waistband to the bottom of the Fell. This is where the apron will lay after you reposition the buckles.

Now with a sharp knife, seam ripper or scalpel cut just the stitches fastening the tabs that hold the two buckles on the right side and also cut the stitches that sew the left strap to the under apron. DO NOT CUT OFF THE LEFT BUCKLE!

Put the two right buckles onto the right side straps using the center hole. Lay the kilt out on a table as if it were fastened on you with the area where the buckles go on the top and centered in front of you.

Bring the outer apron over until it is aligned with the mark you made and with the chalk mark the new position of the two buckle tabs. This is the new location of the tabs and where you are going to carefully sew them back on.

Use a good quality polyester thread, such as Gutterman, in a color that matches the color of the buckle tabs. Use little tiny stitches and try to make them as close to the edge of the tabs as you can. The idea is to catch just a couple of yarns with each stitch and make them as invisible as possible.

Once you have the right side buckles re-attached put the kilt back on with the center hole of the loose left strap through the buckle and also through the hole in the waist.
You will now make a mark on the front of the under apron where the end of the strap now aligns. You will need to push the outer apron down and out of the way to mark the under apron and it helps if you have one of those big spring alligator clips or a real strong clothes pin so you can do all three things at once. (pinch the strap in position to the inner apron, push the outer apron out of the way and make the mark with the chalk.)

Now all you have to do is sew the left strap back onto the front face of the inner apron. (yes, it is going to be moved from the edge of the under apron to some where on the front face.) Make three or four passes of the thread through each hole in the strap. This sewing needs to be strong because this is where the most stress is when you cinch the kilt on everyday.

That's basically all there is to it. Remove and reposition the two right side buckles and remove and reposition the left side strap.

If your kilt has sporran loops on the back you will find that they may now be off center. You can simply cut them off and throw them away. They just help the sporran strap from falling off really skinny hips.

You may also find that the pattern of the pleating may be off center too if your kilt is pleated to the Sett. You can't correct this and will just have to live with it.

But hey, your kilt will fit, you get bragging rights that you did it yourself, and two or three hours sitting sewing your kilt is a lot more fun and relaxing than the same time sitting and watching TV.

ps. If you stick yourself while sewing it's considered good luck.