I don't think the weight of a passengar has nearly as much impact on margins as you all think. The vast majority of the cost structure is fixed: the pilots, the ground crew, the flight attendants, the plane depreciation / maintenace, and fuel for the empty plane. A 737 weighs 91k lbs empty (unclear if that's with or without fuel) and seats ~160 people depending on the configuration. Even if the average weight of a passengar is 200 lbs, the full plane would only add up to 30k lb of people. So, in other words, only 25% of the fuel would be used to actually transport people vs simply transporting the plane itself.
The determinant of profit per passengar is much more about what those passengars pay, hence why they have prioritized wifi over entertainment: business travelers will pay more for good wifi, leisure travelers will not pay more for entertainment.