On full WN flights, when there are a lot of overweight people around, I have deliberately chosen the middle seat between two thin people, even when aisles and windows were left. For a short flight, I'd rather not be pushed out into the aisle or crushed into the window by someone who barely fits into a seat. I have also selected a center exit row seat for some long flights on AA, when the flight was obviously going to be full. On at least two occasions, I sat between a couple that then volunteered to take the center seat.