There also does seem to be a difference in pedaling technique or perhaps in a particular variety of muscular effort entailed by pedaling a heavier bike. It feels very different to my legs, even if HR is the same. My sense is the the heavier bike doesn't "get out of the way" of the pedal stroke in the same way that a lighter bike does. The heavier bike accelerates more slowly, and particularly more slowly during the phases of the pedal stroke. I think it takes some time to get used to. Downshifting much earlier helps as does concentrating on spinning. You have to just spin up every little thing, which on a light bike you might just pop over. You also have to learn to make the weight of the heavier bike work for you rather than against you.