How to clear nextcloud/__groupfolders

Yesterday I did my first nextcloud backup. At the end I was wondering why the size of the backup is much bigger than the origin data. So I walked through the filesystem and I identified some big data which are not needed anymore. First of all I deleted the trashbin. This saved me around 20% of unnecessary data. Furthermore I found one groupfolder which is also not needed anymore. The groupfolder I deleted with my Administrator Account direct in the Nextcloud webview.

When I have a look inside the folderstructer of my nextcloud folder then there is still some GB in the groupfolders. But there is no groupfolder anymore?

16G ./__groupfolders/versions
1,1G ./__groupfolders/trash
17G ./__groupfolders

How can I identify this data in the system and remove all the content to cleanup the groupfolders?