"Worse yet is when that double-wide removes, or persuades you to remove, the chair arm that separates your seats - then that behemoth really spills over into your space. It's not a matter of claustrophobia, I'm getting crushed by their spill-over. " The arm rest is your friend!! I'm just under 5 ft, and weigh 100 lbs. Some of these people act like they have a right to the space that I paid for! I'm sorry they're squished- but that doesn't mean I should be too.
Something that I've never understood. With two seats, there are three armrests. With three seats, there are four armrests (the law of fenceposts applies..).
So it appears that the arilines expect some sort of ritual combat to decide which seat will control the center armrest.