I think the difference is that because the 2 stroke is timed with ports that restrict the length of the stroke, the 2 stroke needs to rev higher to get the same torque as the 4 stroke.

The 4 stroke has an advantage with the use of valves, it can pull in more air and fuel, and will have a longer burn cycle than the 2 stroke. The 2 stroke can use it's higher revs to off set this advantage, but by doing so it needs a smaller prop.

MR Flyer57