Larger opening = more volume = better out of the hole
Smaller opening = more velocity = better top speed.
WITH THAT SAID.........
if you go too small, you can create so much pressure in the pump, that you will bog the engine, and in turn, loose speed. When you start to play with the nozzle, you really have to experiment... and it's never a "Night and day" difference. It's really for the racers who will tune for every race.