I think we are in agreement. Your words "can be" are the key words. It can be, may be even more often than not, but not always a crucial factor. Even within the class A/B group you can find examples where weight do not guarantee more power. Example: Bryston 4BSST/2 vs Emotiva XPA-2. The Bryston is just as powerful but 30 lbs lighter.
The Marantz 5003 weighs about the same as the 6004. I still think HTM did their measurements under slightly different conditions. May be when they did the 5003 they held the supply voltage constant, or something like that.....
Just an aside, if I remember correctly HTM rated the 6004 top pick but not the 5003.