I just want take a moment to show just a few of the potential fit related issues here. It is far more complicated than any single dimension given above.
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Saddle Height (Taking into account differences in saddle, crank length, Q-factor, shoe/pedal combination, pedaling style differences between road & MTB, etc.)
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Saddle Fore/Aft (Related to above, with likely differences related to body position for bike handling, that lead to power variation as well.
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Handlebar Reach/Drop (Relative position from saddle to hands, with includes final Reach & Drop position, as well as width and hand orientation).
All of the above lead to a functional body position that can have a direct impact on the RPE and output from the rider, as well as comfort and injury issues (like the sores).
- Also absent from the OP is sufficient detail about the comparison other than power & heart rate. Questions about the terrain use for each comparison are relevant. Differences in relative pitch and even surface condition can lead to notably different rolling resistance, inertia and relate demand on the rider.
- Cadence is even a notable factor that must be matched if you are using heart rate for comparison. It’s variable on it’s own from all the influences we know, so matching conditions on the bikes as much as possible is the only way to have even moderate confidence in HR as a comparative data point.
Essentially, this is way more complex than can be summarized in a few sentences, and requires lots more info from the rider on the testing and bike setups.