External backup stored locally as well

Hi, Im running a nc server, using an other nc instance on a remote server for external backups, like configured at settings > backup and everything seemed to work fine until I noticed that my 100G storage volume was full, although the synced data was only about 40G. I found that the external backups were stored also locally in the nextcloud/data/appdata_ocz33ektp1ne/backup folder. Why is this and how can I prevent this? I actually used the external storage option to not have to use the local storage volume. Can I delete those local backups without causing damage?? I didn’t find any documentation on this feature online.