I have already found it in the meantime and have reset your usage data for assets.
You have to notice 2 things:
It can take a few minutes until you see it.
The asset usage database is derived from the events, so if a new event like AssetUploaded is raised in the system the usage database is updated. This is called a projection. Sometimes these projections are recreated and then my change is lost and you asset quota is up again. I don’t expect that I have to do it but just in case…