I'm actually having the same problem with this right now.

I'm not using any UI mods and I'm trying to resize my UI from 800x600 to 1366x768 so that my action bars, map, and chat log are a bit bigger... as they are right now there incredibly small.

Every time I switch the resolution to 1366x768, though, and restart WoW/KC, it goes right back to being 800x600.