Even for slim people, airline seating can be a tight squeeze. As a big girl, I'm very sensitive to airline seating. Whenever possible, I get an isle seat so I may insure I do not encroach upon my neighbor's space; I can lean out into the isle, choosing being struck by every hip and beverage cart that passes by rather than crowding you.
Sometimes, despite every effort, I'm stuck in the middle. Usually next to my husband, but his shoulders plus my hips make it impossible to gain much space in his direction. So I pull myself in every way possible to again insure that your space isn't encroached upon.
So it REALLY pisses me off when, instead of returning a similar courtesy, the skinny little bastard beside me slings his bony ass elbows 3-4 inches over the arm rest and into my side for the entire flight!!! To add insult to injury, he sat sideways in the seat, with his feet and legs almost sticking under the seat in front of me! Seriously!?
Being the jovial type, MOST of my flight experiences have been great. My personal space requirements are pretty flexible, so when placed next to someone with similar tendencies to mine, we quickly resolve the issue of, "OMG, my leg touched your leg, did that offend you?" and we get on with just getting comfortable in whatever space we have. The last leg of the flight home, I was sitting next to a bubbly, fluffy girl like myself and her partner, and we all had a blast, hip rubbing the whole way.