The owners manual indicates that the operating range RPM for the 9.9 to be 4500 - 5500 where for the 15hp it is 5500 - 6500 RPM. So where the 15hp gets it's power over the 9.9 is that the carburetor allows more fuel/air to increase the RPM by 1000 which equal the extra power.
The Full-load Torque is the torque required to produce the rated power of an electrical motor at full-load speed. In imperial units the Full-load Torque can be expressed as. T = 5252 P hp / n r (1) where. T = full load torque (lb ft) P hp = rated horsepower. n r = rated rotational speed (rev/min, rpm)