I'm a size 6 in both.. but that's each cheek......

But seriously when i'm not the size of a house, i am the same in undies and pants. I generally only wear bonds though. Where you getting your undies? Are they made overseas? Have you tried smaller undies? I can't imagine a 12 ever fitting you?! Maybe you have that body dysmorphia and you buy a 12 when you really need a 10?