If you have a waist measurement of 50" or more, you will pay for 2 seats, unless there is a spare seat on the plane before you depart. In this instance you will get a refund on the second seat.
With all the weight restrictions on baggage and the exorbitant penalties if you go over your allowance, this seems entirely fair.
Have you ever had your airline seat encroached on by a tellytubby? Not funny!
There is a direct cost to the airline with respect to payload and fuel, so why should someone who weighs 20 stone pay the same as someone who weighs 10 stone?
I can see the airlines introducing a scale of fees according to your weight. If it wasn't so expensive installing discrete weighing scales infront of all the check-in desks, I reckon this would have happened already.