It is easiest to understand when you think about your mountain bike (or your old Schwinn 5 speed :^). The highest gear?or rather the fastest gear is when the chain is driving the smallest gear. That means when you reduce the size of the engine pulley, the supercharger compressor will spin faster and create more boost.

A stock MINI Cooper S runs with .8 Bar boost?roughly 11.75 psi (14.7 psi = 1 Bar?to convert Bar to psi, simply multiply bar by 14.7).

Reduced sized pulleys typically generate an increase in boost of 3-4 psi which roughly translates into 20-25 hp.

Also, there is a noticeable difference in how the engine behaves as the engine will reach higher boost much earlier in the RPM range than stock. Torque figures go up in a similar manner as well.