Chris's guesses are:
1 - Cylinder = 1000cc
2 - Cylinders = 750cc
3 - Cylinders = 675cc
4 - Cylinders = 600cc
5 - Cylinders = 560cc
6 - Cylinders = 540cc

mine are with the simple difference of 75cc

1 - Cylinder = 825cc
2 - Cylinders = 750cc
3 - Cylinders = 675cc
4 - Cylinders = 600cc
5 - Cylinders = 525cc
6 - Cylinders = 450cc

I wish the manufactures would specify megaphones and at least they would sound good even if any sort of formula logic has gone out the window.