I'm basically gonna spit back out history as told my professor, and this is a little long.
The thinness ideal originated in France in 1908, when the French designer Paul Poiret created the Oriental sheath dress. The Oriental sheath dress is sheer and sleeveless and only tall, thin women can fit into it. Poiret was interested in feminism and he thought that the dress would liberate women because it was comfortable and allowed for free strides, as opposed to the heavy, tight restrictive clothing of the Victorian era. Because the dress was made for tall and thin women, it de-emphasized womanly features such as breast and hips. Basically, Poiret created the dress with good intentions. He thought that de-emphasizing breast and hips and allowing women to move freely like men would make them equal to men.
The dress became really popular in France. Then in the 20's, America went to war. When American soldiers were sent to France, American women saw footages of the streets of France, in which French women were walking around in the sheath dress. American women became worried about their husbands and boyfriends lusting after French women and saw the French women as competition. Soon, American women adopted French fashion, and they began dieting to fit into the sheath dress.
In the book "The Body Project," the author Brumberg investigates how women's attitudes towards their bodies developed by reading the diaries of girls from past to present. She found that dieting was a relatively new phenomenon--girls did NOT talk about dieting at all until the 20's. My professor argues that Poiret's sheath dress was largely responsible for the dieting phenomenon.
I know it sounds a bit crazy to say that this sheath dress triggered the whole diet and thinness obsessed culture, but there are other contributing factors including the invention of the television, which enabled women to compare themselves to other women outside of their immediate contact. In addition, there was the invention of refridgeration and vaccuum sealing which created an abundance of food. Because there was an abundance of food, food became a "moral" issue of whether one should and should not eat.