Thank you for reminding about this (it’s been ages since I last touched that book).
The lava planet scene is bad.
I was never a fan of Mustafar, true. In my head-canon I believe this was just a Sith temple (a contrary to Jedha) and his “bunker-castle” is elsewhere and looks just as depicted by McQuarrie.
He was less bulky than Prowse-in-suit, agreed, but I don’t find it glaring (just like I didn’t count how many inches is G.Henry taller than P.Cushing).