Figure 1. The 777X has 30% larger windows than standard in the class. Source: Boeing.
The 777X is best seen as a cross between a Boeing 787 and a 777-300ER. It inherits the passenger comfort features the 787 brought to the market like lower cabin altitude, higher cabin humidity, larger windows, and a smoother ride.
If a 777-9, with it's higher capital costs, can compete on operating cost with a used 777-300ER depends on the fuel price.