The belt shouldn't be considered decorative as much as part of the kilt, although yes a quality kilt should hold itself up. It depends on where you hold the extra weight, if it's mostly lower down the kilt may tend to migrate up not down. Much as I like SWK they tend to be cut for some with no hips. Wear your Stillwater and enjoy but treat yourself to bespoke kilt next time a USA Kilt or Canadian Casual Kilt in a quality PV is surprisingly not that much more than a wool SWK, and it's made for you in your size.