git and github are two different things.

> The [Nixpkgs] repository totals 83GB with half a million tree objects and 20,000 forks. A local clone is only 2.5GB — the rest is GitHub’s fork network storing every pull request branch and merge commit.

> [For CocoaPods' Specs repo] the culprit was shallow clones, which force GitHub’s servers to compute which objects the client already has.

Etc.