This is like a black hole out there… Nobody can tell you exactly the answer. But the answer is not simple at all…
Ok, first of all, there’s a pretty big difference between how the size is calculated for GitHub.com and for GitHub Enterprise.
The short answer is that the size shown doesn’t match the repo’s size after clone. The size shown is as close as possible to the .git folder present in your repo.
I’ve been using git at work for the past few years and absolutely love it. I also use it for my home projects. In all cases I use Github to hosts the remote copies/versions of my projects. Github is a great service but when you have a free account (as I do for personal projects) your project will be public. However recently when I had some personal projects I didn’t necessarily want/care to make public as they were largely PoC or Hello World type projects but I still wanted to use version control. So I decided to set up my own remote git repository. This turned out to be not as I expected.