> Isn't that something that's more the responsibility of the airlines, who decide how many seats are put in a plane, following the limits of the aircraft?
For pitch, yes. For width, no —- you run into quantization pretty quickly. The difference between sardines and standard economy is less than the difference between 3+3 and 3+2 — it’s not really feasible for most traditional airlines to choose a different number of seats across than the design point. And for a 3+3 setup, adding 6” to fuselage diameter makes a noticeable difference to (wider) passengers, which the airlines can’t really take away.