I was going to reply to this thread - but then I thought "What's the definition of 'thin'?"
When I was a size 22/24 I might have answered "size 14 or 16", but now I'm a size 12 (sometimes even a size 10) I don't actually think of myself as 'thin'. Slim, maybe .. curvy, possibly ... but not actually 'thin'
I'm a Cambridge Diet counsellor and saw a client earlier tonight who weighs exactly the same as I do (10 st 11 lbs) and takes the same dress size as me - yet she sees herself as 'fat' compared to her size 8 best friend and is desperate to lose a stone to get into a standard size 10 (she's 5'5" tall by the way).
Maybe it IS all relative - and thinness (as with beauty) is just in the eye of the beholder?