Secondly, the aesthetic quality of the appliances are very different. In ROTJ it really just looked like flesh that had withered away, whereas in ROTS it has a plastic, sculpted look which highlights the depiction of the face not as an "ancient sorceror" whose appearance was due to the ravages of time and power but as a deliberate construction so as to appear monstrous.
I think the way McDiarmid played the role also contributed; in ROTJ he is quieter and somewhat labored, whereas in ROTS he is very expressive and energetic, especially in the end battle where the appearance really looks the most distant from ROTJ. The expressions of the artist under the makeup adds to the way the makeup looks. Thats why the scenes where he played things small and simple resembled the OT character more (ie some of the knighting scene, the scene where he reconstructs Darth Vader).