That is kind of the point I am trying to make. With the crazy US laws, if you make the obese pay for 2 seats, then shouldn't someone that is really thin get a discount? If I am the obese person, then if I pay for 2 seats, they really should be comfortable (straddling 2 seats is not comfortable) seats. If we start going that far, then you might entertain a ticketing by weight basis. However, that brings up problems for the really tall thin people, who really aren't taking up that much more space horizontally (most of what we are concerned with on this thread), but may weigh more than your normal passenger. Really short people that are rather thick for their size would fall into a special niche too! It sounds extreme, but these law suits will happen in the US and from past awards, they will win.
Of course, if you make the seats comfortable for all, then the airlines cant make a profit. I really believe the best way to handle the situation is the way that Gaucho described. I have actually been the beneficiary of a free flight for volunteering to take a later flight. I fly quite a bit and this has only happened once. You are FAR more likely to encounter over-booking of a flight (where the airline actually tried to jamb two people into one seat on the off chance someone doesnt show up)!
Vincenzo