I'm just thinking that size of the character isn't an issue when it comes to accurate design reproduction. I'd think he and Kermit would be about the same level of difficulty. Maybe Robin would be a little less due to his simpler pupils and lack of collar, but then you have the tiny head size that would almost certainly be more challenging as a puppet (because Jerry's hand would have to fit), but maybe not the poser.
On another note, I have been told that Kermit's inner headspace is quite roomy for the average size hand. There's so much that goes into a Muppet performance that we don't really think about!
Right. I understand the issue of room inside the head. I was just thinking about how Robin is alot closer to a basic sock puppet than Kermit is. It just seems to me that once you have the pattern down right (which, as you say, because he has a smaller head, would have to be perfect) he seems like an easier puppet to build. Or, at least, a bit less time consuming. His hands are a lot simpler than Kermit's would be, anyway. He doesn't have those thin, long fingers.
Maybe I'm wrong in that, though. I do see where he could present a problem. Still, he's a lot less complicated than Rizzo and Pepe!
On Kermit's head, when Jim was playing him (and I'm sure you've heard this, too) there wasn't anything in there but Jim's hand. He had huge hands!

Steve has large hands, too, but I don't think they're as big as Jim's looked. You can kind of see that Kermit has a bit more "loose skin" nowadays than he used to.