I can't speak for anyone else, but I don't like the looks of a kilt (or pants for that matter) worn below a large belly. It just looks bad to me. A high waisted kilt will help disguise the large belly. It won't hide it, but it won't look as bad.

I say measure the kilt at the waist, not under the belly.

But again, that's just me (and I have a large belly).