The reason for a two stroke having more specific power (power for a given capacity) has been already been given as been due to the fact that two stroke has a power stroke every cycle vs. every second stroke for four strokes.

Just to clarify some things on torque vs. power.

The formula to calculate power when you have the torque is
Power = Torque x angular speed
angular speed is the speed (rpm) of the motor. From this equation it can easily be seen how 2 stroke engines can get such high power figures when compared to a similar sized 4 stroke motor is because it revs so high to compensate for the low torque figure.

The most simplistic definition for torque and power in our rc case is
The higher the torque, the better the ability to turn a larger prop
The higher the power, the faster the engine can deliver the torque

To make a statement that one is better than the other if one looks at it purely from a torque or power point of view is very difficult (I would go as far as to say impossible). For starting out in the hobby I would definitely recommend two strokes because they are simply easier to run and maintain for a beginner (I know there are exceptions).

To get a good idea of how 2 strokes and 4 strokes work, this is a good page
[link]http://www.keveney.com/Engines.html[/link] to see how this type and a lot of other types work.

Lastly just to add one last comment to really confuse it all, although 4 strokes are definitely more fuel efficient than two strokes in radio control engines, the most fuel efficient engines in the world are large 2 stroke diesel engines.

Cheers
Pups