Figure 1. First flight test Boeing 777-9 with the GE9X engines mounted. Official roll-out is set for March 13th. Source: Boeing.
The 777-9 is a larger and heavier aircraft than the A350-1000. Its wide wing and efficient engines compensate the added weight when compared with the A350-1000. Both have best-in-class fuel efficiencies.
The "cut and shut" 777-8 has the same capacity as the A350-1000. Here the wings, engines and empennage from the 777-9 give it Ultra Long Haul capabilities. In normal route use, this capability weighs on its fuel efficiency.