I disagree with practically everything suggested.

Reducing scope and splitting a single task into multiple PRs each small but part of a bigger picture makes it very hard to see the bigger picture.

You should try to make PRs small, but if a PR is big, then you just have to spend more time to review it.

Formatting commits as a story is a huge hurdle for the one making the changes. And unless every PR is meticulously prepared - going over the commits by the reviewer is a waste of time.

I agree you should return PRs you don't understand though. Or don't feel comfortable reviewing for whatever reason.