Okay, here is the reasoning behind comparing a rotary motor to a piston motor with double it's displacement.
A piston motor's crankshaft rotates 720 degrees (two rotations) for every cycle. Since a piston will only have one power stroke in a single cycle, we can say that each piston will have one power stroke per 720 degrees of crankshaft rotation.
A rotary motor drives an eccentric shaft, however it is essentially the same thing as a crankshaft so I will refer to it as the crankshaft from here on. The rotary motor's crankshaft rotates 1080 degrees (three rotations) for every cycle. However, each rotor has three faces, each in a distinctly different phase of the cycle at any given moment. Each face is beginning the "stroke" that the face ahead just finished, so we can say that each face is one "stroke" behind the face in front of it.
This chart illustrates this.
Here is a diagram stolen from howstuffworks.com to help the visualization.
http://static.howstuffworks.com/flas...-animation.swf
So, since we know that each cycle includes 1080 degrees, we can see in the chart that there will be three power strokes in 1080 degrees of rotation. Thus 3 power strokes / 1080 degrees = 1 power stroke / 360 degrees. So we can see that the rotary motor has one power stroke every 360 degrees, or two in 720 degrees (per rotor). Compare that 2 Power strokes per 720 degrees to the piston motors 1 power stroke per 720 degrees and we can see that the rotary motor is having twice as many power strokes in the same time period and therefore should produce very roughly twice as much power as a piston engine of equivalent volume.
So in order to compare the two types of motors on a level playing field, we double the rotary motors actual displacement to see the volume of a piston engine that would produce the same amount of power. It isn't exact, as there are many differences between the two motors, but it can be used as a rule of thumb when comparing a rotary motor to a piston motor.