I'm an overweight passenger. I weigh over 300#, but due to my build, I generally don't need a seatbelt extender (sometimes do). I have no trouble sitting with the arm rest down for a flight (though if there's no one next to me, I'll raise it). I generally book window just so I have a place to lean into a couple of extra inches without making anyone else uncomfortable.

I don't have a problem with people needing to buy two seats if they actually can't fit in one, but those of us who are overweight but do fit in one seat shouldn't be penalized to that extent. I'm very careful not to encroach on others' space.