How would it work? When I make the booking I decalre my weight? How many people, particularly the overweight, would tell the truth? Or would you weigh everyone on arrival at the airport - can you imagine the arguements that would ensue and the time it would take and if you had to pay for your weight at the airport and your card was refused what then?
For a fleeting moment I dod consider suggesting that seats on the plane could be different sizes with the heaviest people in the biggest seats but then I (1M82 tall) but with a "nornal" BMI would end up in a naarow seat, no thank you. Not only that but the overweight member of the family gets a bigger seat than their spouse or children so they're all over the aircraft, families would not like that.