They don't use git or any other distributed version control system, so there is no incentive to keep it small. And anything outside the source control system isn't accessible to all the tools that use it, so it would introduce complexity.
Nah, because it would cost the company billions of dollars in lost productivity waiting for these files to get re-processed every time someone built the thing. Google's general philosophy is that humans are expensive and computers are cheap, so pretty much anything that helps the humans go faster is a going to be a net benefit in the long run.
They sure do at Amazon. Frugality is one of the explicit leadership principles and initiatives often have cost saving as a primary goal and always as a secondary goal.
You can get an idea of what it looks like by reading the Bazel docs: https://bazel.build/versions/master/docs/be/overview.html
Storing a few text files at Google doesn't cost millions of dollars, BTW.