From a kiltmakers viewpoint here are some random thoughts.

1 The only case I can think of where your hips will be smaller than your waist is if you can back up to a wall and the first thing that touches is your waist. If your butt touches first your hips WILL be the larger measurement.
Remember how a kilt is supposed to fit. The aprons in the front are supposed to drape straight down in front. No puckers, no bulges. The tapering is all in the back. So if when you back up to a wall and your butt touches first your kiltmaker must taper up to the waist.

2 DO NOT USE YOUR PANTS SIZE!!!!!!!!! Kilts don't fit like pants. They should be loose everywhere except at the top straps or waistband and at the Fell or tapered area in back from the butt to waist. Everywhere else the kilt should be allowed to hang loose and vertically.
Pants, and especially jeans are actually sized up to two or three sizes too small. This is called 'vanity sizing'. It is rampant in the world of fashion. Don't believe me? ask a woman.

3 Decide how you wish to wear your kilt before deciding what kilt you want to buy. If you wish to wear your kilt at low rise, (waistband over your hipbones, like jeans) then please don't ask a Traditional Kiltmaker to make your kilt. It is a disfavor to the kiltmaker to wear their kilts in a manner they are not designed to be worn.
If you want to wear your kilt at mid or low rise look for a kilt designed to be worn there.

4 The tip about measuring your hips while standing with your belly against a wall is a good one. If you are a 'gentleman of substance' you must allow room in the hips for the aprons to hang from your belly so measure that way.
And if you are one of these men Please, please try wearing a kilt in the traditional manner with the top straps or waistband at your natural waist. The effect is very slimming. And it's far less constricting.

5 The higher you wear your kilt the less 'slope'. The lower you wear your kilt, the more 'slope'. put on a pair of trousers and turn sideways to a mirror. See that the front is lower than the back? That's slope. Now think of a bath towel hanging vertically from the same place. To keep the hem horizontal to the floor there will HAVE to be slope built into your kilt.

6 The higher you wear your kilt, the tighter it is worn, and will feel. The lower you wear your kilt the more 'ease' must be allowed for. A Traditional Style Kilt is strapped on. It should feel kinda like a kidney belt. A kilt worn lower needs more room because the hip bones don't give so you need some room to breath.

That's enough ranting for one morning. I'll shut up now and go back to my sewing machine.