I don't know if this is true or false, and to me it seems slightly homophobic, but just throwing it out here as it's relevant to the discussion, but I remember reading a discussion of western ideals of beauty for women, specifically to do with weight, in which the fashion industry was blamed for setting these false ideals. The article claimed that male fashion designers - mostly gay - were striving to employ defeminised women to show off their clothes. I can't remember if the article said it was to do with them wanting the models to be more masculine in shape, or just less feminine, nor can I remember where I read it!
But if you think about it, the fashion industry has had a lot more impact on the way we perceive weight in the last 60-70 years than it has throughout human history. And it's the fashion industry that drives this idea about thinness equalling beauty. That's obvious, I know. But there are a few figures in the fashion industry in the west who have really shaped the idea of what the ultimate female form should be like. If you think about it, a lot of our obsession with thinness could in fact stem from just a few influential individuals. But then again, there must be something innate in us that allows us to accept this ideal, surely.
I definitely agree about the idea that it's linked to health and wealth.