This phenomenon is a by-product of society's enormous affluence. In past days, everyone was broadly poorer and had less disposable income to buy rich, fatty food. As we got wealthier, we used our money to buy more calories. Hence, the tendency to eat more fat makes most people chubby. Since "the beautiful" are always a minority in society, "thin" became "beautiful" because it became uncommon.
This is true, but just looking at the 20th century in America, it's never been attractive to be "fat," but it was attractive to have hips and feminine curves. Young women in the 50's used to be called "a carpenter's dream" if they were flat as a board - today they'd get a modeling contract.
(Though I certainly wouldn't welcome the days of girls being made to feel bad if they happen to be skinny, mind you.)
Things have changed even in the past 15-20 years. Cindy Crawford and Christy Brinkley were 80's models when I was a teen and they had attainable figures (with a push up bra...wink wink) as opposed to skeletal ones.
The Practice? Don't get me started on Lara Flynn Boyle or Calista Flockhart. They look like Barbie heads on a stick.
I've got a T-shirt that says "By the time I am thin, fat will be in", LOL!