Changing the jockey wheel size could be a legitimate way to match a given derailleur to a particular cogset. A 13-26 cogset with an 11T jockey pulley would have about the same cog-to-jockey distance as an 11-24 with a 13T jockey. As others have mentioned, a larger pulley implies not only the closer chain coupling mentioned above, but also longer pulley and bearing life and more chain.