Subversion works with large code bases, but not crazy massive codebases.
The version control is just the tip of the iceberg though, and is largely a solved problem: git or subversion, then perforce or straight to Facebook's mercurial stuff.
It's the other tooling that breaks on large enough mono repos that you have hard time with publicly. Searching your code takes awhile. Cross references don't go wide enough or take too long to generate. Builds take too long. Refactoring tools either take too long or don't support the repo of sufficiently large size.
Mostly this isn't a problem though, because few repos are actually large enough to cause real problems.
