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.
The lava planet scene isn’t bad because the lava planet is bad. In fact, that might be the only good part about it.
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).
His bulkiness (or lack thereof) is not why he isn’t convincing.