Indeed, `git ls-remote` produces 14MB output; interestingly, 12MB of it are `refs/pull/<n>/head` as it lists all PRs (including closed ones), and the repo has had ~200,000 PRs already.
It seems like large GitHub repos get an ever-growing penalty for GitHub exposing `refs/pull/...` refs then, which is not great.
I will do some further digging and perhaps reach out to GitHub support. That's been very helpful, thanks Scott!
Have you switched already to the "new" git protocol version 2? [1]
> An immediate benefit of the new protocol is that it enables reference filtering on the server-side, this can reduce the number of bytes required to fulfill operations like git fetch on large repositories.
