When my husband and I were both younger and narrower, we had the middle and window seat on a full plane. A late boarding passenger eyed her aisle seat, flipped up the arm rest and sat on me. I flipped up the seat between me and my husband and sat on him. We ended up using 1 1/2 seats, and my seatmate used up 1 1/2 seats. It was a four hour red eye, and I developed a kink in my back on that flight that still twinges at time. The thing is, she paid for one seat and we paid for two.
I fly a lot, and this was not the last time it's happened, but I am at least prepared. I request an aisle seat, and if I'm in the middle, I don't allow the arm rest to be popped up. The seats on planes are incredibly small these days, and I want as much room as I pay for, the seat back reclined, my foot space, the area between the arm rests.
In India, the guy in front of me didn't want me to put my back pack under his seat, and in Estonia, I had another Slavic woman sit on me. Just last week the guy in back of me pushed his under seat suitcase so far forward it was under MY feet as well (I asked him to remove it....). I'd rather put up with smoking (still allowed in some parts of the world) to bulkhead seats, and not having my full paid for space.
On another note, I really appreciate when there is some effort by the larger passenger to accommodate me. I've given up my aisle seat, and arm rest, to a passage who is having a hard time. But I've also paid extra for larger seats myself on airlines like Fin Air that has unbelievably small seats, especially for overnight flights.
It's what you do, sometimes.