I have chronic back problems and cannot stand a coach seat for more than 2 hours when I have to use the foot space to store my carry-on (enforce the rules airlines!). I also admit that I am overweight although I can fit in coach without infringing on my neighbors space - even on the more and more common RJs we use at smaller airports (Regional jets).
This is a complicated issue. The airlines cram more seats into an airframe for revenue reasons, while the U.S. has a population that is larger than ever and I don't mean in numbers.
Canada's courts ruled their airlines couldn't do this for flights within their country.
The airline's have a right to define their passenger policies within FAA regulations.
As for me - if the flight is going to be longer than 2 or 3 hours I try to book first or business class. I know this isn't possible for everyone but you have to pay for it if you want or need extra room.
I recall a flight a few years ago where a morbidly obese man was seated at an exit row and needed three belt extensions (we were directly across the row from him. He had bought two seats, but I was alarmed to see him by the exit and the flight attendants did nothing about it. That exit was essentially useless. To his credit, the man said he was headed to a facility in Florida to undergo treatment for his girth.