Having been in a middle seat between two obese people spilling over their armrests
I can definitely appreciate Southwest's position. But the seats are so darn small that the last time I fit comfortably into one, I was about 12 years old - and I'm not all that tall or heavy. How uncomfortable must it be for someone who weighs 300+ lbs to wedge themselves into that space?
I agree with the idea of "oversized" seats that passengers can request for a small fare increase. Say, seats the size of first-class seats, but located in coach and receiving the same service level as other coach passengers.
I think one of the criteria that Southwest is using, is whether the passenger requires a seat belt extension. Other than that, it's up to the ticket agent or gate agent.