Flying with assigned seats, I request the aisle seat, as a broad broad. I unapologetically request a seat belt extender from the attendant first thing. Sometimes, if there is not much of a crowd around, I request one as I pass upon entry. I smile and am polite. I do not attempt to fasten my seatbelt until I obtain the extender. I pull the armrest up before being seated.
When I get the extender, I fasten it, but only after all passengers have been seated in my row.
If I am flying Southwest where seats are not officially assigned except A, B or C, I am careful to log in early as possible before leaving for the airport in order to register for Group A which enables me to be seated during the first group. I choose an aisle seat, and like above, I request an extender, waiting to fasten my seatbelt until everyone has been seated in my row. While I am waiting for Groups B and C to be seated, I make myself appear as fat and unhappy as possible. I frown. That's all. I make eye contact with every person who walks down that aisle, and, using my very best "Resting "bit*&face" expression, magically, no one chooses to sit next to me. Easy peasy.
Never apologize for your size. I don't like have half my thigh flopping over into the next seat, but I am wider than a 140 lb person. So what? The airlines choose to make their seats 17 inches (barely) and no one is "comfortable" during a flight. You have a right to be there just like the tiniest person does. Some people talk incessantly, some have horrible breath that knocks you out from 10 feet away, some drink heavily, crunch chips or nuts in your ear, some people need a bath or don't use deoderant. Others fart. We are each different. There are a number of things that make me physically or mentally uncomfortable when I fly. My extra girth is the thing that makes others uncomfortable with me. I am aware of it. I wonder if they are equally as self-aware of their own idiosyncrasies.