When she put the kilt on, the underapron edge now actually wraps around her 10.5" before the strap goes through the kilt buttonhole.
When I lost that much weight and moved the buckles on my old kilt, I took that excess underapron and folded it under at the buckle. It seemed to swing better that way while I was dancing. This was on a 12oz tartan so it didn't add too much bulk, but I would imagine that folding under wouldn't be much of an option with a heavier tartan.