to everyone who responds with "it's proportional to square footage": nobody flies planes configured for business class only. "if the moon was made of cheese we couldn't mine 3He from it" but in reality it isn't so why should I accept this assumption?

@whitequark square footage is not the correct metric, obviously. But first class seats are heavy. A brief web search not only gives me figures of 135kg to over 200kg for the luxury variant which already nearly doubles to triples the weight pp, in some cases airlines even need to add counter weights to the back of the plane. Add things like showers and the additional water for those.

These are extremes, of course, but this can add all up.