Because the smaller carb has smaller diameter venturi, the carb is more sensitive to changes in vacuum signal coming from the cylinders via the intake. This gets the carb drawing more fuel more quickly.

The smaller volume of air in the venurii, (and in the intake plenum) allows the air to "accelerate" more quickly = more responsive.

Conversely, running a large carb and an intake with a large plenum, can move more total volume of air, but it takes that larger volume of air longer to "accelerate."

Hope that's technical enough...