I'm 5'7", 169lbs and a size 16, although depending on the style of top I might be size 18 on top.
I agree that there appears to be a huge amount of variation, another reason why the BMI scale often appears to be 'off'. I suspect a lot of it is down to build. I am quite broad shouldered and big boobed so I always feel like a heffer no matter what weight I'm carrying, I've always envied that narrow shouldered 'bird like' figure. It's difficult to talk about differences in build though without people assuming I'm making excuses for my size or weight which is very frustrating. I would probably not mention it anywhere else but here!
Clothes manufactuers are starting to face up to the fact that you can have a 'petite' frame but still be a size 20, which is a start.
It just goes to show that weight loss is a personal journey and comparisons with other women really aren't very helpful.