Tourist. Thanks for your erudite contribution. Unfortunately, you are incorrect. The same is true in airliners - heavy ones take longer to descend.
A call to the ETPS chaps may put your mind at rest. For a given Nr, a heavier helicopter has a lower RoD. It is a fact. Go and give the tefal-heads at Boscombe a call. If they disagree - I'll delete the thread. Some very good theories on this thread by the way - thanks to all the contributors.
Tourist - you are wrong. I just don't know why.