If you are over a percentage, like 15% of what should be your average weight then you pay more and have to be seated in larger seats.
How about letting the marketplace decide?
Airline A offers the lowest price, but has small seats for everybody.
Airline B charges by the pound, same size seats for everybody.
Airline C charges by the pound, but the seats are different sizes.
Airline D charges one high price with large seats.
We have different cars, houses, restaurants. Why are all the airlines in a race to the bottom?