CF6 and RB211 were of modular design, where (N1) compressor first stage (the big fan) could be removed in the hangar. The rump was then of manageable bulk to fit the cube of CL-44D/707-320F/DC-8F.

If you take the LP fan off a RB211 it doesn’t make it any smaller as it sits inside the fan case.

You take the nose cowl off it to make it shorter though.