A 13B, with 1308cc of displacement, at any given RPM will inhale (given 100% volumetric efficiency for comparitive reasons) the same volume as a 4-stroke piston engine of double that capacity, 2616cc. Simple as that.
Think back to Mazda History 101, where using Mazdas way of thinking they "forgot" (could have something to do with Sport car import duties) one major thing, that they are dealing with a positive dispalcement engine NOT a gas turbine. They have selectivley neglected to count 2/3rd's of the engine when they quoted their capacity of 1308cc for the 13B.
Next time you get the engine built you can leave out one apex seal from both the front and rear rotor and see how well the engine will run. It will still have one chamber per face that will give the 1.308lt (it will only do this 1 out of every 3 revs) so you should have full power - in theory. The point is you need ALL 3 sides of each rotor to be functioning for the engine to function and as such you should count all the sides when you are defining the capacity
phew...