@whitequark @rcombs it's going to depend a lot on what engines are on the plane, but in general bigger airframes have bigger engines and bigger engines use more fuel

they don't necessarily use proportionately more fuel, though; in absolute terms a bigger engine uses more fuel, but it produces more thrust, that additional thrust means it can carry more weight, and that means more passengers and cargo. if the increase in revenue from passengers and cargo outweighs the increase in costs from fuel, it makes sense to use the larger plane

or in other words, if both a larger plane and smaller plane can carry your passenger and cargo load, the smaller one is likely to be more economical (but this is a really complicated discussion because it needs to consider things like flight time, fuel burn per hour, what cruising altitude each plane has...)

@demize @whitequark I'm largely setting aside differences in the engines themselves in favor of assuming they're mostly going to be similarly efficient in their design deployments these days, but compare eg: a 737 MAX 10 carries ~33% more passengers than a MAX 7, has ~12% more MTOW, and has 72% more cargo space, but weighs ~13% more empty; same engines

those numbers pretty clearly add up to "the bigger one is more efficient on more-popular routes, while the smaller one is more efficient on less-popular ones"