> A passenger takes up
a seat regardless of how large he is
I'm only halfway joking when I say I've lost many an elbow into the side of someone substantially larger than I. For some "a seat" is "their seat + several inches of mine".
I know it's not really the problem at hand, but it's poor proxy for volume, and again, just shows that the weight fee is just a sham.
Just going to start manufacturing a "flight jacket" with pockets for all your luggage...