If a massive PR can't be broken up into multiple small but meaningful sub-PRs then it's normally good manners to say something like 'here's what I want to change, this is why it needs changing, and I think it needs this massive overhaul which touches these million and one things'. But I don't think that's a likely scenario unless a codebase is poorly designed and the prevailing wisdom of 'to make a difficult change: first make the change easy, then make the easy change' will usually be applicable.