So, what's the deal? Was Jabba originally supposed to be a human character, or did they just use this guy to give Ford somebody to interact with during shooting? I'm not nerdy enough to know.
Pretty sure this was a bit that was cut from the original, not a stand-in. So as of at least the filming of this scene, Jabba was supposed to be this guy -- He of the Massive Double-Chin changed his mind somewhere down the road. And when the episodes were re-released they put the scene back in with a teeny-tiny CG Jabba pasted on.