Within my experience, many if not most airline seats are made for people under 5' tall. That's just one more thing airlines need to think about if they don't want to become a footnote in technological history. The human race is not getting smaller and the world did pretty well for itself in the decades of fast ships and zeppelins. I could live with that at least as easily as I live with airlines and their attitude problems.
It's interesting when the Alaska Airlines plane flies from LA to Seattle. The people are so small, there is plenty of room. Then the little people in their little business suits get off at Seattle and people in Carhartts get on whose heads brush the cabin roof and shoulders hang over both sides of the aisle. Off to Alaska! where 6-3, 240 is not considered large.
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! ;-)