I've always thought that there should be a base price for all (say $50 for an average flight) and then you and your luggage should be weighed and you will pay an additional fare based on the total weight--$1 per pound might be about right.
So a seven year old with 25 pounds of luggage and carry on might pay an additional $85 while the 350 pound businessman with 60 lbs. in checked bags and carry on would be charged an additional $410.
The seats would be the same size for all.
I work hard to keep my body fit and travel light, so I'd make out like a bandit.