Originally Posted by Terrierman
Makes sense to me. Tires and aero are about it for the big differences no? Assuming similar gearing. Though MTB riders tend to ride on smaller frames, which would also effect the equation, along with the heavier weight and loss of some efficiency coming from that front suspension fork.
I think you are underestimating the importance of the geometry differences in a road frame vs. a MTB frame (not just size).