Oops. You beat me to it...
You had a good point about the rich driving body image. Being tan used to mean you did physical labor outside. Now it is a status symbol, thin and tan.
But some trends do come from the poor, the whole rap/hip hop culture came from poor people and was embraced by fairly well off white kids. I just have a hard time imagining pop stars being generally obese some day though.
Freegards