I have seen (and recently bought) entry-level rear derailers from Shimano (Tourney for MTB, 2300 for road bike), and I keep wondering why they are making these parts with such large pulleys.
I don't think this make the parts cheaper to produce, since large size means more material, and I can't see why customers would find this more attractive or adequate than the smaller-sized ones (unless customers would prefer more expensive models because they DON'T HAVE large pulleys...).
Also, I imagine there might be advantages to larger pulleys (less chain link rotation, less wear, slower pulley rotation), but the reason why entry-levels are big, while more expensive models continue to be the same old size, is very unclear to me, not to say arbitrary.