Small repositories are great for open source because they encourage code sharing. When your project consists of many pieces all owned by different, often volunteer, teams, then having one big repository is a barrier to that.

For example, Servo, the project I work on, consists of around 150 small repositories (in contrast to every other browser engine). Lots of them are maintained by the Servo project, but lots of them aren't. They're separate projects of which Servo is only one user of many. The fact that we can take advantage of the fantastic work of the Rust community by simply adding a couple of lines to our Cargo files has been invaluable in helping our small team get the browser off the ground. A culture of monolithic repositories would discourage code sharing, leading to more wheel reinvention and less code collaboration overall.

I think the model works well for Google and Facebook because they're big centralized companies with thousands of engineers under one management and reporting structure. But that's a far cry from the aggressive, fine-grained code reuse culture that the Ruby, JS, and Rust (for example) have fostered.