The quota includes not just the git repo, but also other services that gitlab offers. In particular the container and package registries are easy to setup in a way that accumulates a lot of data (e.g. by building and tagging a docker image in every commit and not cleaning up old images). In most cases this will be a misconfiguration and all the quota enforcement is doing is to force people to configure their projects a bit more considerate.

As far as I know, the quota itself isn't new, it just wasn't enforced in the past.