I think the best single observation about cognitive load is in Ousterhout's book A Philosophy of Software Design[1]. In the book he promotes the idea that classes should be "deep", such that their top-level surface API is small relative to the complexity they hide underneath.

This applies to the microservice/monolith debate as well. And it basically boils down to the observation that having lots of shallow services doesn't really reduce complexity. Each service may be simple unto itself, but the proliferation of many such services creates complexity at the next level of abstraction. Having well designed services with a simple API, but hide large amounts of complexity beneath, really reduces cognitive load for the system as a whole. And by "simple API" I think it's important to realize that this includes capturing as much of the complexity of error handling and exceptional cases as much as possible, so the user of the services has less to worry about when calling it.

[1]: https://www.amazon.com/Philosophy-Software-Design-John-Ouste...