I think this observation applies to software development generally, not just open source projects. In "The Mythical Man Month", Fred Brooks talks about the concept of "the surgical team". One thing he stresses is the idea of having a single, overarching vision of what the architecture should look like. This becomes harder to achieve as more people are added to a project. More opinions are tossed around and debated. Lots of people have lots of good ideas, but not all good ideas are compatible with each other.