I should probably be somewhat careful what I say here, but I work for a commercial software company with one of the largest perforce databases (according to perforce) in existence. 3000 developers working on several million lines of code; I don't know how many MB offhand, but I'd guess O(1GB). Our central perforce server is a monster that we literally cannot keep fed with enough RAM, yet we frequently wait for a minute or two for a simple p4 submit, or p4 sync, to complete.

git on my local machine eats this tree for breakfast. I seceded from p4 a few months ago, and my workflow has never been smoother; I only ever interact with perforce for actual checkins, and to pull in other developers changes.

I guess the moral here is that to talk about whether something "scales", we need to be clear about what dimension we're trying to scale. In our experience, trying to get perforce to scale to huge numbers of developers has taken a lot of effort. Since git is numb to the number of developers, it has the potential to work better in our environment. YMMV.