I have only sat next to a horrendously fat person once on a plane and it was awful, but despite the amount of times I've travelled, I only recall this one time.
Plus, I think he had the attitude of 'these seats are so damn small it's alright to sprawl my arms and position my body in such a way I squash that woman next to me'.
My pet gripe now is when people try and tell me they have the window seat, when I always book it ahead. I've had several people argue the seating arrangement with me, the 'oh, I was here first, so I can pick any seat I want in this row' attitude.
I had one really argumentative woman once, totally holding everyone up because not only was she in MY window seat, she was in Seat A when she should have been on the other side of the plane in that particular row. She starts raving, unprovoked that I can just swap with her seat on the other side, she aint moving. We are talking over nearly 5 minutes of arguing and even the people behind her and in front of her were telling her she had the seat wrong and she had to move...so, patience ran out and I zapped her with,
"Listen, this is an aeroplane, not a cinema, your ticket has a number for a reason, not least of which, if the plane crashes, they will be looking for your body in the correct area. Now, it would be fantastic for you and I if you move out of my seat before someone calls the hostess and you are kicked off the plane before take off as a risk of aggressive behaviour."
She moved.