Hello,
I've been lurking around, enjoying reading this board for a while but this is my first post - hope you're all well!

I recently went from 11st 1 to 9st 10 - starting off at a size 12/14 and am now a 10/12. So that was 18lbs = one dress size for me! I think its definitely more to do with your underlying shape rather than your weight (although that obviously has something to do with it)! I'm an hour glass shape with really wide hips

- my hip bones actually stick out (but thankfully my chest is also a decent size and balances them out) and I don't think I'll ever stand a chance of fitting those hip bones into anything less than a size 12 no matter how little I weigh!
On the other hand I have a friend who weighs about the same as I did originally and she fits easily into a size 10/12. She's a totally different shape to me - with small hips and thighs (very unfair) and carries all her weight on her stomach and chest. So, I figure it's definitely something to do with your body shape and not just weight.