Yeah, sounds like the alternator isn't putting out enough, whatever the cause.
Yes, a 2g pulley is smaller, so it'll drive the alternator faster.
It all has to do with rpm (of course, you know that), so note whether they only dim when stopped at a light and idling @800-1000 rpm? Or do the lights also dim when driving at 2000rpm+?
Was the alternator new or used? It's possible that the regulator has gone bad. Since you don't want to a) waste power with continuous high load on the engine and b) overcharge the battery, the regulator as I understand it has the power to disable coils in the alternator so that it only puts out as many amps as you're using. You only want it generating 5 amps (just for example) on the highway in the daytime on a mild day with no fans or lights on; while if you're stopped a light with the radio on loud, HID headlights on, etc. etc. that's when you need the 130amp capability (which is not 130 amps @800rpm, or with underdrive pulleys and 800rpm, mind you)
There are two things I'd try.
First, do you know if the alternator's good? If it's new, rebuilt, or, before you installed it you took it to an auto parts store and had them test it, it probably is. An alternator can put out 13.5V, but not be able to keep up the amperage output.
Second, like the poster before me suggested, check the lights themselves. Pull out a wiring harness for one of the lights, turn on the lights, and with a voltmeter check what you're getting.
My car is 20 years old, and the headlights have like 18 gauge going to them. I found out that I was only getting 10 volts to the headlights! No wonder they were dim for me. A relay setup so that they get power direct from the battery with a much lower gauge, fresh wire, will return full voltage to them.
But finally, yeah, you have maybe underdriven the alternator too far, and yes, the 2g pulley will spin it faster. That's the most likely, honestly.
