If overweight people should pay more, then underweight people should pay less. All of my children weigh less than an average adult yet I had to pay full fare for all of them when we flew. Two of which could comfortably sit in one seat without hindering anyone. We could make it so that when you boom you give your mass, then you are re weighed on entry to the aircraft and fined on the difference.
Or we could be civilised and charge per seat.
