Once I sat between a couple of guys going to a pro football training camp to try out. It was miserable. They were nice guys but I could barely breathe. I'm 6'1" and about 200 lbs. I felt like a runt.
The airlines regulate the size of carryons so I don't see why they can't regulate the size of the passengers if they, the airlines, insist on making one size seat fit all. If I pay for a seat then I expect one seat, not 90% of a seat or half a seat. The airlines are the the cause of this problem, not the fat passengers.