This pulley is already smaller than the stock pulley, though? Not sure I'd want to try an even smaller one...
The alternator is much smaller, so not the same speed of the armature moving past the magnets. The vehicle that this alternator was originally designed for would have had a bigger drive pulley I would think. Have you had a volt meter on it? Mine shows 11 volts or less at idle, and this is the second one, both the same.