Stearns is quite specific in timing the change, however, pointing to the years between 1890 and 1910.  In these 20 years, he writes:
…middle-class America began its ongoing battle aginst body fat.  Never previously an item of systemic public concern, dieting or guilt about not dieting became an increasing staple of private life, along with a surprisingly strong current of disgust directed against people labeled obese.
I thought of Stearns’ book when I came across a delightful collection of photographs of exotic dancers taken in 1890, the year he pinpoints as the beginning of the shift to thinness.  From a contemporary perspective, they would likely be judged as “too fat,” but their plumpness was exactly what made these dancers so desirable at the time.
More at Retronaut.