Here's my problem with the idea of charging by weight...
I am overweight. I still fit in a regular seat and use a regular seatbelt, but I am 100 lbs over my ideal weight. I have no problem paying more for my seat because I am heavier.
However, my husband is 6'6" and weighs 250-260lbs. He has very little fat, and is all muscle. By the charts, he is considered obese, but in fact he just has a large build and is very healthy. So why should he have to pay more to fly just because his body is so heavy. The charts say he should be 210 lbs, and he would have to be quite sick to get to that weight.
In order for this to work, they would have to figure who is overweight by choice, vs. those who are by lifestyle. It would never work.