Airlines are in the business of selling the transportation of volume and mass. Persons or items than exceed a necessarily-standardized volume (and/or mass, but mass is not usually the issue with passengers, as opposed to their volume) must be charged for the added volume being transported/ displaced.
I feel for folks who are bigger, and I “push it” myself (6’1” and just under 200#, I find it difficult to jam my elbows between the armrests), but there must be some upper limit “per seat.”
I agree on both points. Sitting next to an obese person is not good. However, at 5’ 8”, 170# and a 32” waist, the seats are damned uncomfortable on flights over two hours. International flights of ten hours plus are agony.