That's fair. When you send a package via FedEx you pay by the pound; why should human cargo be any different? I weigh 160 lbs. and I don't want to subsidize some glutton's plane ticket. My daughters weigh much less. Why should they pay the same fair that someone three times their weight pays?
It's hard for me to believe that your suggestion has not gotten more play. It's the only sensible way to charge passengers since weight equals more fuel burned. There should be some form of basic rate and then a sliding scale for every pound a person weighs. It would even encourage a weight reduction incentive for many of us.
It should be true for baggage as well. Pay by the pound with a premium paid if you want to make those pounds of baggage carry-on. We'd be spending a lot less time waiting for people while they try to stuff their entire wardrobe into the overhead compartments.