I'm 5'3 and 9 12, have lost 2.5 stone over the last couple of years.
I think sizing depends enormously on your shape and where you shop.
I am a 12 really, but can be a 10 on the bottom, at the moment. I am a small build and an apple, small bum and hips but medium boobs. My weight is all on my torso basically on a small build, so even though I'm definitely a bit fat (for me, I used to be 8 stone) I can get into smallish clothes. I am aiming for 8.5 stone which will be about right for me, I always used to be fairly teeny.