the diameters of the tube and the raise at which air passes the sensor are totally independent of each other. the one intangible you did not mention is although the pipes is smaller the restriction towards the nose of the car is severely decreased by comparision to the stock air box, ergo reguardless of the diameter air flow increases dramatically. look at it like this.

if you have a funnel with a 12 inch cone, and a 1 inch expelling aperture, it will not releasing its contents as fast as a funnel with an 8 inch cone and 3 inch expelling aperture.

its a rough comparision but the dynamics are similar