Economy seats on the plane will be "over 17 inches," Reuters says. By contrast, comparable seats on the A350 will be 18 inches. The extra inch equates to one less seat per row (10 in 777X and nine per row in the A350).
Ultimately, it's all about efficiency, as the 777X will purportedly carry 406 people versus 350 for the A350. As it turns out, the difference is a big deal, and sets the stage for what could turn into a testy ad war ahead of this month's Dubai Airshow.
Airbus contends that the extra inch "improves sleep quality by 53%." Here's BA on the matter: "It really comes down to providing flexibility to airlines and allowing them to do the things that they believe they need to do to be successful."