i'm 5 foot 4, around 13 stone 13 and a size 16 in most shops ie Dotty P, New Look. can get into some Next 14s but still can't get into 16s in places like Oasis, Warehouse etc. I own a size 14 dress from H&M but went in there a few weeks ago to try and get a pair of work trousers and strugged to do up the 20, was gutted and slunk out in a sulk.
So I have come to the conclusion that clothes sizes suck bum. Loads of places do 'vanity' sizes and it's not what you weigh, it's your body shape that matters.