Yup, the larger pulley will make fewer revolutions compared to the smaller one, spinning the alternator slower. Although that would make charging worse at low RPM’s, which can already be an issue with the stock alternators
Keep in mind not all the crank pulleys were the same diameter either. The larger alternator pulley, paired with a larger diameter crank pulley, wouldn’t necessarily be under-driven. You have to look at the ratio of the diameter of the alternator pulley to the crank pulley.
