The airframe designers don't decide the seat space per passenger. They can include all of the spacious reclining seats and lounge sofas as they want in the des

The airframe designers don't decide the seat space per passenger. They can include all of the spacious reclining seats and lounge sofas as they want in the design, and airlines will spec it with seats and seat pitches that produce the greatest profit per flight by maximizing the number of passengers within the limits of safety regulations.