But surely first class passengers have paid for the extra "real estate" that they are occupying. After all, isn't that one of the reasons that they are paying double or treble the fare of the ecomomy passengers.
It's only paid for at "normal" (ie pre-surcharge) cost of fuel. When the cost of fuel goes up, then the extra cost atttributable to each premium pax is a higher number because each of them has (say) 200 kg of aircraft attached to them rather than the (say) 100 kg of aircraft attached to each economy passenger. (Numbers plucked out of the air, but you get the point.)