She's a nice girl actually so I am probably BU. But I can't believe she doesn't realise what she's doing (ie fishing for 'ooh but you're so slim remarks).
She is always saying 'Ooh I really need to lose some weight for the summer/for my holiday/whatever ... I'm feeling so fat at the moment' or words to that effect. She must have said it three times this week including this morning on the school run - there's a little gaggle of us who stand around together in the playground. She is very slim and fit (about my height, 5ft 10ish) and can't be any bigger than a size 8 at most, and is clearly confident in some ways about her looks as she usually wears tight fitting or quite revealing stuff (ie very short shorts in the summer, skin tight jeans, etc). I am not big, size 10/12 so I'm not being bitter and twisted 
but there are women she says it to who are probably a lot bigger and it just makes me cringe.
Some of them used to automatically start telling her how slim she was, how she doesn't need to worry, etc. But she still keeps going on and it's getting annoying. I would love to think of something clever to say next time but I don't want to sound like a jealous cow....