Airline seats aren't exactly comfortable, but you'd need to be pretty damned fat to really hang into the next guy's seat. I can recall flying into rochester mn, and a few people on the little plane were using belt extenders to span their Midwestern guts, yet they weren't really creating space issues for the people seated next to them.
On a similar note, during my commute home yesterday I was on a packed subway train in Boston. For anyone not familiar with the Red Line in Boston, there are long banks of seats with priority seating at the ends for the old, disabled, pregnant, whatever. When I got on there was a very old woman in one of the seats, and a very obese man across from her in the other, along with the seat next to it. The train was packed, so I generally appreciate the obese guy being seated where he's less of an obstruction to the flow of people on and off the train. I think that's completely in keeping with the concept of priority seating fwiw.
Anyway, we go two stops to a connecting station (south station) and a very obese woman pushes through everybody at the entrance going on about "excuse me! I need to get to my seat!" Only to encounter the two priority seats taken, and no real room to squeeze in amidst the other seats even if one person got up. And one lady did get up and offer her a seat, but she decided to go on with some minute long excoriation of the rest of the commuters instead of attempting to fit in one seat with no end wall to pack into. I get she's not in good health, but there wasn't much anybody could do to make her comfortable. Nobody had much sympathy when we're already packed in and it's mid heat wave.
Now, she could have attempted to get to the other priority seats, but that would have required maneuvering through a crowd on a train in motion. And nobody seemed all that interested in playing human Tetris for her given how rude she was to the crowd. She got off at the next stop, so this all went down in about 5 minutes.
I guess the point being, when the system's already set up to pack as many people in as possible, some folks are always gonna be uncomfortable, and that should, naturally, include the obese as well. The person on the airlines should have compacted his/her giant ass into one seat, you bought that limited space, it sucks, but you gotta respect the shared discomfort.
Even the giant fat guy in the picture seemed willing to compromise for his neighbor's sake.