One thing they could do is calculate the total weight of the PAX plus their baggage. For example, I was 3Kg over weight (20Kg allowance) checking in for a TCX flight last September. The check-in staff pointed it out, but let me off saying I may get charged on the way back. The two people behind me were almost obese - as a guess I would say he was about 3 stone heavier than me, and she was probably 1 stone heavier. That's about 25Kg in total more than my entire suitcase!
So if you're a larger person, you either take less, or pay more - after all you are using up more space and more fuel.