As a plus sized woman who often flies coach, I honestly feel that if someone is uncomfortable next to me that I would be happy to move to a different seat. But honestly if the fat person is uncomfortable, maybe they should buy a first class ticket. Although i'm plus sized, I have sat next to a person who was over 500lb's and that person expected me to squish into the window seat while they took up the other two seats (that person did NOT buy the middle seat) I refused, and the guy started calling me a b**** and things like that...the nice stewardess moved me to first class.
While I hate to make people uncomfortable I always get the isle seat in order to prevent making anyone else uncomfortable