I have been reading Rethinking Thin: The New Science of Weight Loss--and the Myths and Realities of Dieting, by Gina Kolata.
One of the things she mentions early in the book is, right around the turn of the 20th century, upper-class woman would corset themselves into 10-inch waists, then carefully cultivate an image of illness and frailty. The fashion of being invalids who were sickly and swooning all the time, pale and thin, was unattainable for working-class women, who needed functional bodies to survive in the real world.
A "functional body" is one that is not corseted to a diameter of 18". For a woman to have the strength and endurance to work and raise her family, she needs to eat and breathe well. A woman who has servants and wealth may have the luxury of voluntary debility, but for such a state to be a desirable trait that a man would seek in a potential spouse? He isn't interested in a healthy, happy partner; he wants a living decoration, a waiflike doll to control and use. Working class men did not have as much "luxury"--they needed strong, healthy spouses for their families to succeed, and therefore needed to accept that they would not have as much control over their wives.
The craze for thinness, especially in women, has its roots in keeping women subservient to men. If we are hungry, undernourished, and sickly, then we are easier to control. Fat is a feminist issue.