True but you never know with Luffy
Your points are true but the reason why I think Luffy is not going to pull large scale attacks is his battle against Noah. He was desperate about destroying it so if he has attacks with more DC and larger scale then I am sure he's have used them back then. And if he really has much stronger attacks and didn't use them to destroy Noah then it would be bad writing on Oda's part.This chapter gave me an impression that Zoro would have destroyed it much easier than Luffy. May be Luffy's haki will evolve during his battle Joker.
Btw, How much bigger do you think Noah is when compared to Pica's golem?
As for the size comparison, not sure yet. I'd have to scale it and i'll give you the results(if i can), but for now i'd say Noah is probably around 3times bigger from view just, so not perfectly accurate