I feel like collaboration can work great with a group of exactly two people. It's not terribly hard for two people to partition work and actively help each other. With two people working on a project, both people can realistically understand most of the codebase, and can competently review each other's pull requests.

I feel collaboration suffers from combinatorial complexity though, and I feel any number bigger than two ends up doing more harm than good. Once you have more than two people, the codebase starts becoming more segmented, it becomes really difficult to agree on decisions, and the project becomes a lot harder than it needs to be.

If I ever get into management, I think I will try and keep this in mind and try and design projects around two-people teams.