Explained to me by a type rated CX engineer in MEL handling UA 789, greater range of the 9 over the 8 comes from Aerodynamic Slots on the vertical and hori stabs that improve boundary layer flow over the rudder and elevators respectively and hence aerodynamic efficiency.
No magic tricks other than that and as previously mentioned fuel capacity relatively unchanged.
No research on my behalf whatsoever.
I'll leave that to my pending type course.