There are TONS of factors when talking about the efficiency of the drive system of the accessories. Not only do you have friction, but interta, thermodynamics, and when you thow in the alternator, now you have electrical losses too. There are also minor losses due to aerodynamics, bearings, tension, ect.
In practice there are two things that can help the efficiency of this system the easiest. #1, reduce the inertia by reducing mass of the pullies and nything else that rotates.
The #2 thing is the radius of those said pullies, but that has a muliple effects. It reduces intertia of the pullies by reducing the mass. It also will slow down the speed at which the accessory turns. In this case it will reduce the losses of the electrical system of the alternator, but there is a point of diminishing returns with that, especially at idle. With the reduction in speed you get less thermo losses due to friction as well as from the bearings ect.
Unless this system is VERY inefficient, then the best way is to simply reduce the mass of the pullies. The ONLY reason I run an underdrive pulley on my car is that I do a LOT of highway driving. I wanted to slow the accessories down to try and extend their life, and to possibly eek out a tad more fuel economy out of it.