There are many varying shapes, sizes and qualities of pulleys. Most pulleys made for low end fitness equipment are made from cheap, brittle plastic with a low quality bearing ring pressed into the center. Pulleys commonly found on “Commercial Grade” equipment are made of premium plastic polymers that contain 25% total fiberglass mixed in prior to the molding process or aluminum at the top end. A larger diameter causes less stress on the fibers of the cable going around them increasing the life of the cable as well as making for a smoother overall cable system.
Commercial grade pulleys typically have high quality, precision bearings inserted into the pulley at the time of molding so they are precisely placed and cannot shift or come out. Cheap pulleys have low grade bearing pressed into the center later in the process. Of course the quality of the actual bearings within the bearing ring make a big difference as well. Depending on the particular application a wider flanged pulley can prevent friction when the cable you are pulling goes off center. Putting the pulley on a swivel mount also help alleviate this problem.
At Bodyforce we use 115mm diameter pulleys every chance we get. We also use custom made aluminum pulleys to ensure the most fluid cable movement.