Hoshnasi's comments seem fairly correct in terms of what we are talking about, at least in my experience and what I perceive. Let's skip over the semantics of "low surface area" and not argue that point of whether it is the correct term.

A tire/wheel that contacts the road less is generally going to be the smaller tire overall. With that smaller amount of material, a road bike tire is going to have some advantages in less air resistance than your average MTB tire will. The smaller tire is also going to have less friction in contacting the road than the MTB. In theory, the more a tire contacts the surface, the more work is going to be required to counteract that friction. An unlimited source of power will be able to generate more speed the bigger the surface area is (to a point), but cyclists are not unlimited in their leg stregnth and will work harder in general using a MTB tire.

A road bike tire is also usually going to be lighter than a MTB. In surges, the lighter tire is going to be easier to spin up to speed. More weight, no matter how insignificant, usually means harder work up hills and getting up to speed.

Mountain bikes are usually less aerodynamic in position. An average MTB has the rider at a straighter sitting position. A road bike has the cyclist leaning forward and cuts down on wind resistance. Many of the MTB's out there run with frames that are heavier than a road bike frame. More work = less speed in the long run when the muscles start getting tired.

The biggest factor, however, is the engine. Two exact people will ride at the same speed on the same bike. Put one on a road bike and the other on the MTB, and the road bike is going to pull away. A super cyclist on a MTB will outride an average cyclist on a road bike. An average cyclist will fall behind on a MTB against a super cyclist on a roadie.