THIS. The fat bopo movement likes to whine incessantly about the right to take up space, which sounds great in something like a doctor's waiting room. (My medical system of choice has ALL bariatric chairs at this point.) But where space is severely limited...
Spoiler: Dumbass SW sperg
I took SW recently, and the two very last people to board were massive, and sat in my row. I had an aisle seat. Both required extenders on their seat belts. I got smashed into the aisle side armrest, to the point I was sitting at an odd C shaped angle. I said the one next to me, I can't sit like this for hours. He wouldn't look at me, and told me he didn't design the seats, or load the plane, IT WAS THE AIRLINE'S FAULT.
After we took off, I called over an FA, who blamed me for not saying something before we took off. *I* had to be the one that moved, so I sat between two slightly less massive people. I had to cross my arms the entire flight, because there was no room for my shoulders.
Mx. Comfyfat won't give you a traveling tip, but I will. One of my parents travels a lot, and uses this technique:
When you are seated next to someone who spills over into your seat, go to the nearest FA, and state calmly: "I'm not sitting there." Usually works, and the problem is now the airline's.
If they insist on flying to SF, there WILL be a lot of whining, anxiety and accusations of fatphobia. Can't wait!