Mixed feelings, here.
I think it's going to be more a matter of how much space you take up than how much you mass - airliners are designed using an "average" weight per seat (much like elevators...) plus a margin of safety, since there are quite a few people who blow the average (6'3" 255# here. I need to lose a little - getting back down to 240-245# would make me happy. No, I don't take up two seats, but I take up all the legroom in front of me and I have fairly wide shoulders...)
Yes, if you travel with a lot of luggage, you should pay extra. Travelling light is an acquired skill - I spent about five years helping my wife acquire it (I can live for a month out of a duffelbag.)
If your arse don't fit in one seat, you should be required to buy two and use two - for the comfort of the people around you. If you can fit in one seat without being absolutely huge (say, 300# and a 36" waist...) there's no real reason to make you pay extra. (Fortunately for me, I tended to fly MAC when I was running a fighting weight of 275-288#...)
Having been seated next to beached whales, I can definitely get behind the idea of a "fat bastid tax" of making you buy another seat - it's not for your comfort. It's for the comfort of people next to you.
The main reason I don't fly commercial anymore? Not enough legroom. Even as a knuckledragger, I still have to deal with "Can you move your knees?" from the guy in front of me. No, I can't move my damned knees, we're talking capability here, not willya/wontya. Just accept the fact that I'm as uncomfortable as you are and suck it up. If I just have to fly, I try to get a bulkhead or exit seat, but you can't get those in advance anymore. They have to "see you to make sure you can help people out" - but when I've gotten on the plane, those seats usually end up filled by some five-foot-tall, eighty-five-pound woman and her two children under ten... Funny - I thought that was illegal anymore? At least for the exit aisle...
Should oversize people pay extra? I'm inclined to think so. However, the issue would then be applying it fairly. There are some people who are just "built big" (me, the OP, the Scottish National Footy team, caber tossers, ...) who can't help it, and shouldn't have to pay extra for genetic predisposition.
Then, there are the people who should have been on a diet for the last five or ten years - that's the problem. If you've got a 50-inch waist for no good reason, you should damned well have to buy two seats. Or maybe walk/swim to your destination? That should help...
This leaves aside the whole issue that does give me headaches - the yoyos running airport insecurity. It was farcical before, and it's gotten worse. I avoid airports wherever possible anymore - I'm allergic to idiocy, bureaucracy, and asinine regulations (they tend to make me break out in fits of violence.)
Anyhow, if you've got someone who would spill over into the next seat, having them pay a "fat bastard tax" of buying that seat should help. Make the seatbelts a little more extensible - you'd fold up the armrest to make one wide seat, use the outer end of the seatbelt from each side, and everyone is more comfortable as a result. Requiring the purchase of aisle seats would be a good step - most of us can't "suck it in" far enough to let them out when they need to run to the head...
