Thanks! Yeah I plan to get to the bottom of it. I will probably propose to just keep a branch with full history somewhere (we need to keep history for auditability) and reset the main branch from a recent state.
Have you already tried a "gc --aggressive"? It's not exactly fast or cheap, but some repositories are very badly packed and only a full reset will fix them.
An other useful high-level option is git-sizer (https://github.com/github/git-sizer) which tries to expose a few useful trouble spots, there's not much that can be done if the repository is just big (long history of wide working copies with lots of changes), but sometimes it's just that there are a bunch of large binary assets.
This may be more likely if the repository was converted from a centralised VCS where storing large assets or files is less of an issue, likewise the bad compression. Though obviously removing such large assets from the core repository still requires rewriting the entire thing.
That won't shrink the repo. Any reference will keep all the objects alive and they all get packed together. If you only care about reducing clone size see this post:
If it helps, I wrote a very long and detailed blog post several years ago about the techniques I used to rewrite my team's Git repo history (including stripping out junk files, _and_ actually rewriting source file contents via formatting and codemods for _old_ commits):
I can recommend git-filter-repo instead, it's relatively recent and there is a lot of outdated info on the internet about cleaning git repos. The --analyse flag will generate a report about files in your repo even if they were deleted. I used it to cleanup a number of repo and it helped in detecting large files commited by mistake 10 years ago.
The history rewrite removed the files and we didn't need to create a new repo (old history still works fine).
This looks like a great tool. I'm not sure if I haven't come across it before or I'd forgotten about it.
In my experience you'll have references to the commits in a repo from outside of the repo: links from Slack, Jira, other repos, etc to specific commit IDs. When you rewrite history, all of the commit IDs change. That's why I recommend archiving the original repo so as not to break any such references. Create the new repo, either rewritten or seeded from the old, in a new location.
It would be neat if git supported a "rewite map" to allow it to redirect from one revision to another, sort of like how `git blame` can be configured to ignore revisions.
An other useful high-level option is git-sizer (https://github.com/github/git-sizer) which tries to expose a few useful trouble spots, there's not much that can be done if the repository is just big (long history of wide working copies with lots of changes), but sometimes it's just that there are a bunch of large binary assets.
This may be more likely if the repository was converted from a centralised VCS where storing large assets or files is less of an issue, likewise the bad compression. Though obviously removing such large assets from the core repository still requires rewriting the entire thing.