Has a lot to do with genetics too.
Look at a young woman, who is not fat (ie, bangable)
Look at her arms.... do they seem kind of soft and chunky... almost disproportionately so compared to the body?
If yes, chances are she'll be a big fat mess when her metabolism slows down. It's pretty common around these parts.
Of course, 'bad' genes means you have to work to keep in shape, not just use em as an excuse and do nothing.... which I guess ties in to tyhe above posts...