I don't believe that all people do this. My group of besties include Danielle (super-fit beautiful tattooed girl), Ashley (wheel-chair bound free spirit super feminist), and Rudy (8th grade best friend, he used to be overweight and now he's a chick magnet), I'm the token chubby chick. I love all my friends and everyone else for who they are.
I see people's reactions to physical differences in people on a daily basis. I work with developmentally disabled adults. When we go to the doctor or on a daily outting, people stare.