From the information I have, primarily the alternator spec sheet I ran the numbers regarding alternator pulley size & rpm and alternator amp outputs at each. These numbers are based on my 55 Amp alt.
1. With engine RPM at 800 and a 2.75 inch alternator pulley, the alternator revolutions are about 2837. The alternator is putting out close to 40 A.
2. With engine RPM at 2400 and a 2.75 inch alternator pulley, the alternator revolutions are about 8507. The alternator is putting out 55 A.
3. With engine RPMs at 800 and a 4 inch alternator pulley, the alternator revolutions would be 1950. The alternator would be putting out about 31 A.
4. With engine RPM at 2400 and a 4 inch alternator pulley, the alternator revolutions would be 6685. The alternator would be putting out 55 A.
Sooo, if one goes to a 4" alt pulley, at close to idle power/landing phase of flight, the alternator outputs would be in my case about half. Like I've mentioned, I'm VFR only so not an issue for me. If you are IFR coming in through the clouds and have low engine/alt rpms on final approach make sure your full current demands can be met with the larger alt pulley. Yesterday when I taxied out after replacing the alternator, (old one was 60 amps), at idle power I turned everything switch in the cockpit on to see if it could handle it which it did. I know a couple years ago it would not have because I had high current draw lights. Now I'm nothing but LED's and they take almost nothing!
EDITED: I am a numbers kind of guy and can't help thinking of weird stuff sometimes. It makes perfect sense that the life of an alternator with a 2.75 pulley would be shorter. The "extra" rpms in a one hour flight are something like 109320! And for one hundred hours of flight time the extra alternator rpm are in the neighborhood of 10,932,000!!!
I figured the alternator I just removed having lived a 475 hour life turned about 237,000,000 RPM's over its life. I know useless information but interesting.