Hey, that was my 2c!
Light aircraft, bottomed collective to maintain auto Nr, get a certain rate of descent.
Heavier aircraft, holding some pitch to control Nr in the green, blades are now at a more efficient angle and therefore ROD is lower.
Rotorfossil's 2c was quite similar, or perhaps that should have been 2p in his case.
I suspect that the answer may be that the higher weight requires a higher lever setting and therefore higher angle of attack to maintain to maintain the same Nr. This moves the L/D of the whole rotor disc towards its optimum conditions. The same reason that max range in autorotation is achieved generally at the lower end of the RPM band.