I just got back from a quick trip to Germany to see a long-lost friend. Atlanta to Stuttgart is 9 hours (make it 10 from the time you board till the time you leave the plane.) I'm no taller or heavier than average but the lack of leg-room in coach was really uncomfortable after the first five hours.
My German friend understood, calling it "rache der Zwerge." Literally, "revenge of the runts."
That's the advantage of first-class; not the width of the seats but the distance to the one in front of you. No way to upgrade at any price, though, as every seat each way was filled.
Airlines are trying to maximize revenue by packing as many paying bodies into each flight as it will hold. If they're at fault, they share it at least partly with a flying public that demands jet travel at a fraction (in adjusted dollars) of what used to be charged 30 years ago.
Having said all that, if the passenger takes up two seats, he/she should pay for two seats.
Of course, tort lawyers will have a field day trying to extend the Americans with Disabilities Act to 39,000 feet. Then the airlines will make covert attempts to find out the body mass of ticket-purchasers, and for some the flight will be "all booked up."
Then begins the real fun! ;-)
Try 13 - 18 hours on a Pacific rim flight in a Northwest airliner (Northwest tries to ensure that even small kinds of people like Koreans feel cramped).
It may really be time for the human race to disabuse itself of the idea that anybody needs to get from Detroit to Seoul in one day and go back to fast ships and/or zeppelins.
The Hindenberg was a hell of a thing and it turns out hydrogen had nothing to do with its final demise; it was the aluminum oxide outer paint. Zeppelins were vastly more efficient than jetliners. They flew with diesel motors, could land anywhere, and offered spacious accomodations to their customers. No paying customer in those days would tolerate what we get on airplanes for a second. Cruising at 80 mph without having to stop at lights, they'd get you to most places in a perfectly reasonable amount of time without the nervous-system damage caused by present air travel.