Just as people make assumptions about being old, so do they make assumptions, usually very painful, false ones, about people who are fat, especially women. Women who do not fit the heterosexual male ideal are considered, at the least, defective and, in the extreme, lazy and stupid.
Fat people are the last group against which people can discriminate with total social sanction.
And look at the size of the women in major roles. With a few exceptions such as Camryn Manheim or Rosie O'Donnell, most female role models in this country right now could also vie for the position of anorexia nervosa poster child. And don't think this phenomenon is limited to heterosexual women. I know from personal experience, as well as from conversations with lesbians and from reading an amazing amount of literature on lesbians and body image, that although lesbians may be slightly less likely to buy into the male standard, many of us continue to carry all those misogynist messages from our years growing up in this society. Lesbians don't like to admit to having body issues.