There is another side to this ...
 

Spare a thought in these situations for the person who is a little on the large size.

World Champion Tasmanian Axeman David Foster is 6'4" and, according to some sources, weighs in at around 28 stone. You can perhaps more readily appreciate the implications of this when you see a picture of him in action at his web site:

davidfoster.com.au

David is a very entertaining speaker with a larrikin sense of humour and frequently uses opportunities to play upon his impressive physical stature and/or his proficiency with the axe. He apparently takes significant delight in the inevitable rise in tension whenever he ambles into a departure lounge and everyone else realises the possibility that HE could be allocated the seat right next to them :eek:. There is often a hasty if hushed re-examination of boarding passes by anxious fellow travellers to see what the possibilities might be.

David goes on the say that when the boarding call is made, he usually waits until most of the others have entered the aircraft before he joins toward the end of the queue, and eventually wanders down the aisle checking his ticket and the seat numbers on the luggage lockers. He senses the latent anxiety, and knows that anxious eyes glance toward him - especially those who have realised that the seat next to them is still unoccupied. He also is aware of a certain sense of significant relief as he goes on past those people who had viewed the vacant seat next to them with suppressed horror at the implication that this might be "the one".

He sometimes makes a point of walking right past the seat he knows he is allocated, and with a finely honed sense of the relief the poor soul feels - ("Phew! he is past me, thank goodness for that .....") - to turn back a second or so later to break the happy news that "Well, this looks like it is my seat ....." :E.

Great bloke - in every way! :ok:

That argument also applies to all children and given that there are many many more children fly than obese passengers, I think the answer to your question is that the airlines just would not consider payment by weight as they would probably lose out.