So make the airlines advertise the size of their seating and leave it up to the customers to decide if it's worth it to them. It's not fair to force the airline to offer valuable space for half the normal price based on a customer's weight. Should the airlines be forced to remove a row of seats so tall folks can have more legroom? Obese people have a choice (some may debate that I suppose) while tall people don't (ok, there's surgery for that I guess).
How much extra should be charged for a wheelchair?