According to Boeing "With new breakthroughs in aerodynamics and engines, the 777X will deliver 10 percent lower fuel use and emissions and 10 percent lower operating costs than the competition.
The 777X has folding wing tips. The longer wings allow for more efficient in-flight operations, and folding them on the ground means it can access more airports.
The 777X is closer to the capacity of some four-engine aircraft. The A340-600 (the largest A340 variant) offers a two-class capacity of 440. The 747-8 has a typical three-class capacity of 467.
|The GE9X engine used on the 777X is bigger than the 737 fuselage|
See video covering of the first flight of the 777X here