Really? I thought the fuselage was all carbon fibre as well?
The fuselage tube is still aluminium. It is being built using a newer improved alloy that contains lithium. Aluminium lithium alloy is slightly stronger so quite a few parts will end up a few percentage points lighter. Alumnium lithium closes the gap between the current aluminium and carbon fibre when it comes to strength to weight. So even though the 777-9 is 3m longer the tube part itself will probably weigh the same as the 777-300ER.
The 777X wing is significantly larger with a heavy folding mechanism. But as the wing is made out of carbon fibre the weight increase will probably be fairly minor. Same deal with the engines, significantly larger diameter but as it is made from newer materials it will reduce the weight increase. Even the shorter 777-8X is predicted to weigh more than the current 777-300ER.
As the 777X maintains the current maximum takeoff weight the 777-9X when fully loaded will carry less fuel than the 777-300ER before hitting maximum takeoff. The reason the 777X will still be able to fly further is the larger wing provides a better lift to drag ratio meaning less thrust is required to keep it cruising. This combined with more fuel efficient engines should see 10-15% less fuel consumed per hour. So if the 777-9x takes off with 5% less it should still be able to fly 5-10% further.