I think this is a big question, and has some varied answers.

On the one hand, there's a pattern of curvier and then thinner women being in fashion. Some say this corresponds to women's movements being public, so:
1920s: Suffrage, thin flapper ideal
1960s: Women's Lib, Twiggy

There are other examples. Some say it has to do with economic trends. You can see that easily, too.

But there's a larger issue, which seems to me a permanently growing hatred of fat people, by which I mean fat, not curvy. And that began at least in the 19th C, as far as I know.

There are great books--Hillel Schwartz, Never Satisfied: A Cultural History of Diets, Fantasies and Fat (NY: Free Press/Macmillan, 1986; Anchor paper 1990), Paul Campos, The Obesity Myth, Kathleen LeBesco, Revolting Bodies?: The Struggle to Redefine Fat Identity and many others--that might help think about this question. No one is perfect, of course, but each makes interesting and important arguments...