Back in the days when I still had cable, I used to watch this airline reality show. I forgot the name of the show - but they featured Southwest Airlines, I think. Sometimes they would have a passenger that they would call "a person of size" and if the people who issues the boarding tickets notices your large size, they will have you sit in the plane seat and see if you can fit into the seat comfortably and properly without making other people who have to sit next to you uncomfortable, then you don't have to pay for a 2nd seat. But if you are so morbidly obese that you absolutely cannot fit into your seat with the armrests down and you make the other passengers uncomfortable, then you have to pay for a 2nd seat.