pulley size is by design ...if the unit installed is for the system then the pulley is designed so.

alternators n generators connected directly to RPM display an output curve directly related to RPM.

so when the maker writes unit's power output is 120 amps (Ford) what is described is the unit outputs 120 at 60+ mph.

this in reality doesn't cover radio esp 400 watt rig/lights/AC in slow go commuting with extensive idle...in cold weather. That 120 doesn't charge the batt

My van has a 200 AMP with an Odyssey Bat. With everything running at 65 mph, 200 amps doesn't charge the batt. Gotta shut off the stereo for a charge. Over uh 4-500 miles. Runs down slowly.

so if you have a problem with this either a new batt or a higher output alternator and a complete check on wires and connectors cleanuo is in order.