Charging by weight would not work. DayvidB, you are male so why should a 5 foot 110 pound woman have to pay extra for you? She is smaller and thinner than you. In fact they should make a plane with smaller seats so she can be comfortable.
Say a women was traveling with her 8 year old son. The woman is 200 pounds and her son sitting next to her on the plane is 75 pound together the weight is 275. The average per person is 137.5 that is a lower weight than most men. How do you charge them?
Unless everyone is weighted and charged per pound it doesn't work. However that has problems to. For example if a really heavy person who normally buys two seats anyway is charged per pound how do they charge the person for the second seat? Does first or business class have a higher per pound price? What about the extra weight for someone's wheelchair is that added? Most important is the scale out in front of everyone?
Cause I can just take a train or a ship.