Ok I'm gonna take a shot at this but, I'm not great at explaining things so try to stick with me. First we need to establish the constant, we'll say the plane weighs 10 pounds so thats how much thrust we need. Now lets talk horsepower, simply put horsepower is torque X RPM. If we spin a 18/8 at 7000 rpm to develop the 10 pounds of thrust we need the prop will absorb a given amount of torque. In order to develop the same thrust with the 18/6 we will need to spin the prop faster. Now here is where it gets confusing, in both case the HORSEPOWER absorbtion of the prop is close to the same {torque X rRPM}, but since the RPM's are higher with the 18/6 the torque is lower. For the prop to absorb the same TORQUE with a lower pitch you would have to increase the diameter, this would add to the "disk" area of the prop and result in higher thrust. I hope this helps
Now I need a nap.