Kilt straps have an adjustment range that allow for a fair amount of weight change.  But when you run out of holes, the buckles and/or the straps themselves can be moved, in or out, up to 2" depending on their current placement and length.  Adjustments larger than that may require that the aprons be adjusted.
 
Also fill out and add to your order the No-Cost Measurement Form to provide new dimensions for your kilt.