My butt fits fine in the airline seats, it's just my damned shoulders. I always try to get an aisle seat so I can lean out into the aisle for most of the flight except when those damn drink carts come by and whack the shit out of my shoulder.

Although the last time I flew somewhere, I got stuck with a middle seat. I had to sit hunched in on myself because I didn't want my shoulders smushing the people on either side of me. Always helps in those situations to try and strike up a conversation and that way you're not awkwardly jockeying for position with the person next to you for the whole damn flight. Luckily the people on either side of me were relatively small women so I wasn't really taking up any space that they were using.