Having done flight plans for trans-pacific flights, back when the 747-100's could only fly 8 hrs, without landing, weight is very critical. I used to call the gate to get a "kid count." If I needed more help, I'd ask for an "oriental count." I know, that's so un-PC but that was the 70's & 80's and the little people weight adjustment make a difference. That didn't work too well with the Manila flights because they carry 2x their body weight in baggage. So, based on that, obese passengers should be paying a higher fare.
Weight is still money! That''s why everybody is flying with minimum fuel. Fuel=weight, and reducing weight saves money. If there are any un-forecast enroute winds or weather, you're going to make a "complimentary" fuel stop somewhere.