gitlab.com reporting incorrect repository size
A git repository size on remote includes loose objects, whereas if youre working locally (or off of a fork), you likely dont have those loose objects, or they were cleaned up.
Most of the time, you only have a copy of the default branch and whatever branches youve worked on locally, not every branch that has been pushed to the remote.
You can follow the GitLab documentation on reducing repository size.
The comment from a Backend Engineer on this repository cleanup issue may also help explain whats happening a little further. The key thing from this comment is that you need to wait 30 minutes after following the documented instructions for the Housekeeping/cleanup job to work properly.