"I have long felt airlines should simply charge two fees: one for volume (i.e. economy/business class, etc.), one for weight (person plus all bags, checked or carry-on)." Couldn't agree more. More often than not I have found myself paying for a seat with more legroom only to find myself next to a person twice my "width" and weight and significantly impacted in my comfort because of it. If say the statistical average https://www.cdc.gov/nchs/fastats/body-measurements.htm would serve as a baseline for charging more if above, perhaps that might work.