I started at 15st 7lb, a size 20 (I'm very much hips and boobs!) when I got to my lowest at 12st 4.5lb, I could get into size 14 trousers (very, VERY tightly!!) - I have very strong thighs due to a childhood as a gymnast and trampolinist - and now at 12st 11lb I'm 14 in normal jersey-top t-shirts (not button shirts!) and 16 is coming up just a little big in trousers. However dresses and such I am a 14!
Changes so much from person to person, even when I tell people I was a 20 they are shocked, as I looked more like a 16.