One of the big reasons why models are super skinny is because being shot with the wrong lense can make your features appear much fuller and fatter (hence TV adds 10 lbs to your weight idea). This is especially true compared to normal human stereoscopy vision.
But also its just good business to have practically impossible standards. Which is terrible of course.
I consider myself extremely lucky I've never had to deal with weight problems. My older brother went up and down all the time in middle and high school, which often made him miserable.