Because air can be compressed the flow rate "cfm" has to be in reference to a pressure drop. The higher the pressure drop the more air in cfm that will flow. In an engine that pressure drop is created by the piston moving down on the intake stroke, you have atmospheric pressure (14.7 psi) outside the engine and zero inside or a 14.7 psi pressure drop. If you have ever driven with a vacuum guage hooked up you will have noticed that at idle and part throttle cruise you will have a relatively high vacuum reading. This is caused by the restiction of the closed throttle blade. At WOT the vacuum will be near zero because the restriction has been removed by opening the throttle blades but the carb venturies and boosters will still present a small restiction so you will have the samll vacuum reading. From a max power standpoint you want as close to zero as possible at WOT to maximize the amount of air/fuel that is drawn into the engine.
Because 4bbls are used in performance applications where the goal is to have as close to zero intake manifold vacuum at WOT as possible they are rated at a pressure drop across the carb of approx. 1/2" Hg which is close to where they are used. 2bbls are typically not used in perfromance applications and some restriction is not as important and are rated at a pressure drop of approx. 3"Hg.
What this means is that a 2bbl rated at 300 cfm will require you to suck harder (or blow harder) to get the 300 cfm than you would with a 300cfm rated 4bbl. So that 300 cfm 2bbl is only flowing about 1/2 the air as the primaries on your 600 cfm 4bbl.