This is the reason why girls during my time are averse to biking - they will develop bigger legs that would not be good for a fashionable girl. I also believed in that but when I was an adult, I had a chance to ride with a group where a doctor was a member. In a pit stop, I asked that doctor of his opinion about the big legs of a biker. He said no, there is no proof of that. However, he couldn't say if there was a study debunking the myth.