I haven't ever been on an airplane, actually. Can't say I know that discomfort.
I usually sleep on airplanes, but it's pretty uncomfortable to try and sleep when there's someone's fatty leg seeping through the space between the seat and the armrest and it touches you no matter how far you scooch your own butt to the left of the seat. It's not pleasant. And it doesn't matter how skinny you are. The fat just keeps coming for you.