Making a kilt bigger is usually not easy and moving the straps and buckles will be unlikely to work unless there is already extra space on the under apron

Extension straps are one way of gaining an extra inch or two but you will almost certainly notice the straps because they do not actually make the kilt itself bigger, just extend the aprons out of alignment, so the kilt will still LOOK too small

Many kilts have extra fabric hidden inside but to find and use it is not a task for the unwary. Unless you have experience of sewing it might be better to take the kilt to a kilt maker for alteration. They can give you a better idea of what is actually possible with your kilt