well...wait a minute...that was tricky...Both sheaves are loaded equally in that system. and because you are splitting the forces on the main line tied to the object, this system results in a 3:1 purchase.

So I am changing my earlier assessment....both sheave diameters will affect the system friction equally. Increasing either one will have an equal reduction of system friction. Different sheave diameters will not affect the purchase of the system...larger sheave diameters only reduce the friction because of the larger radius the line turns, more bearing surface, etc. That's my final answer.