The folds on each side of the apron and underapron serve three purposes. On the outer apron and underapron edges, they form the facings so that there aren't any raw edges showing on the underapron and so that the fringe edge can enclose a second strip of fabric for the double fringe (and give the fringe edge some stiffness and stability). On the inner apron and underapron edges (adjacent to the pleats), the fold serves as a pleat that makes it possible to walk comfortably in the kilt without having the apron and underapron "pull". And third, many kilts are made with a little extra fabric in all three areas so that the kilt can be altered and made bigger.

Having said that, altering a kilt to change the size is a major undertaking, and a kiltmaker who does it properly should charge you a bundle. It involves removing the buckles, straps, lining, and top band, unstitching the both edges of the apron and underapron (because the aprons need to be altered symmetrically to preserve the center stripes), remarking a larger size apron and underapron, and putting the whole thing back together again, basting, and pressing. Basically, you're re-doing everything except the pleats, steeking, and stabilizer, and there's the time involved in taking the whole thing apart to begin with. If it's done properly, a kiltmaker ought to to charge $150-200 in labor for such an alteration (at least half the labor cost charged by custom kiltmakers for a new kilt).

And, depending on how a kilt was made in the first place, it might not even be possible to do this - depends on where a kiltmaker started cutting out the pleats and how much extra fabric there is in the deep pleat, the inverted pleat, and the facings.

Bottom line, if you can live with the underapron showing at the fringe edge, move the buckles and straps or buy strap extenders.