Tough call. Just like any other shipment/transfer of item, the more it weighs - the more UPS makes me pay.
On the other hand, you tossed in the idea of paying more for baggage - and thats a whole new curve ball. I'm curious, what does everyone think of that? I can hear all the women with bats and signs raging right now about paying more for their blowdryer, straightener, and curling irons for every trip already and it becomes a sexist battle 😀
So far, the only thing I can safely say after that point is this: For the comfort and satisfaction of both parties, I can't say there should be a "fat tax". However for the satisfaction of parties next to them, it shouldn't be allowed for creeping fat to cross lines. That's just disgusting. Forcing to pay for 2 seats I can agree with for that reason. Pay by the pound now makes it about luggage, coats, babies, and anything else that has weight.