I'm sure such an idea would cause a lot of resistance, and I'm even not sure if it's legitimate considering the quite extensive discrimination laws in most countries. And it wouldn't be fair in all situations, some people are just fat because of diseases, disabilities or other medical reasons. I think with such things in mind this idea is not feasible.
However, being egocentric sometimes I embrace this idea because I would be flying really cheap
Two things are infinite: the universe and human stupidity; and I'm not sure about the universe. -Albert Einstein