Another reason for the RRs being a tad heavier in the CF6 and RB211-524 engines is in the wall thickness of the casings from the LPC all the way back to the HPT. This helps in longtidunal rigidity of the engine. Yet, RR copied GE by using the thrust link arrangement, (aluminum pipe) that ties the aft eng mount to the fwd eng mount to increase stability even some more.......go figure?
Rapid throttle movements aggravates longitudinal bowing of the casing and should be avoided.
Also for the Rolls, the IPC drum and it's associated compressor blades and stator blades including the drive shaft and associated bearings/housing and spoked support struts weigh more than an extra N2 compressor disk/blades and VSV linkages as in the twin spool GE/Pratt designs.
When twin spools can do the job more efficiently why 'over engineer' the engine by adding more spools and it's associated hardware as RR do, making it more complex and costly from a manufacturing and maintenance aspect, yet achieving a similar thrust output.