Just happened to catch a random bit of information on the local news a few minutes ago, comparing airline policies towards overweight people. One policy that actually struck me as fair and reasonable was from Southwest -- if a passenger is too big to fit into one seat, the passenger is required to pay for a second seat, but the extra money will be refunded if the flight isn't overbooked.
In other words, if you're taking a seat away from someone else, you should pay for it. But if occupying an extra seat didn't matter, then the extra seat is free.