I am not morbidly obese (hey I've lost a bunch of weight), nor do I smell (well at least no one has said so, hahaha), but I often do feel sorry for the folks sitting net to me. I am a 6" tall, sturdily build (read big bone, big frame) women of dutch decent. I am not the tall stick figure model sized person. I got both the tall genes and the big boned genes, one from either side of my family.
I fit in an emergency row seat with non-movable arm rests, but just, however my shoulders/arms are likely going to be just a tad bit wider than the seat itself. I always try to get an aisle so I can lean a little away from the person next to me. But not being an average size on airplanes where average is getting smaller and smaller it seems, I always feel bad about even slightly infringing upon my neighbor, yet I don't feel like I should have to buy a second seat.
I will also tell you that not every seat belt is the same length. While I have no issue with 99% of the seats I have traveled on, occasionally I have encountered a "what the hell happened to the seatbelt" seat. Where it must have broke and they repaired it but now it is 4 inches shorter than most every other seatbelt on the plane. So seatbelt extenders, while perhaps embarrassing, do happen to those who generally don't need them.
So what is there to do????
I have heels on in this photo, so I am likely 6'3", you would never guess I was that tall from this photo.