There was an article posted on here a few months ago about Google's version control system. One of the main features is that you don't actually checkout copies of every file in the repository; instead they're faulted in as you need them.

My impression is that the monolithic repository only works because Google's built a bunch of custom tooling to support it. It doesn't mean any company can just roll all of their code into a single Git repository and call it a day.