Don't forget that power is proportional to the torque multiplied by the speed of rotation.
Imagine he same engine, fitted in either a fixed wing or helicopter. Using easy numbers, the rotor of the heli will be rotating (say, 200 rpm) at one tenth the propeller speed (say, 2000 rpm), so will produce 10 times the torque. Turbine engine fixed-wing operators can demonstrate this themselves on the ground; Set prop speed to max, and torque to around 80% of max. Now slowly reduce prop speed and watch the torque rise; watch your red lines!!! The multiple of torque and rpm will remain the same.
Edit (RTFQ!!) : So yes, its because the heli blades are longer, broader, etc. Nothing to do with weight; just has more drag than a typical prop.