We encounter this exact same "problem." We have a few hundred surveys, all of which have photos in them which consume a lot of storage space on the server. And unfortunately, just "deleting" the photos through Esri isn't good enough.
Under the hood, the ArcGIS Data Store is just a PostgreSQL DB. And for surveys (or any hosted feature services with sync enabled), Esri typically treats them as a "versioned" dataset, where you have your base tables along with the associated versioned tables.
When interacting with the data through Esri, it's always using the versioned view. So for example, if you're viewing the data in ArcGIS Pro or in Portal, you're looking at the versioned view of the dataset. And any modifications you make, including deleting the data, get reflected in the view, but not necessarily the underlying base table. If I delete a record in Pro, that will be reflected in the view, but not the underlying base table.
This is where it becomes a problem, because it gives you the impression that the data is deleted, but it actually isn't. Unfortunately, this is where it becomes difficult to fix the problem because Esri doesn't really have any out of the box tools to deal with this.
Several years ago at the UC, someone from Esri told me that you can disable sync and then re-enable sync capabilities of the hosted feature service, and that would get the changes you made in Pro/Portal reflected in the underlying base tables and that would truly "delete" all the records. While I believe that is technically correct, it also corrupted the hosted feature service for us (I think because we had a huge feature service and it timed out or something). So we don't use that method.
The alternative method is connecting directly to the Data Store DB directly through PGAdmin. From here, you can delete the records in the base table.
However, deleting the records from the table does not actually decrease the table size or give the space back to the OS. The final step you have to do is run a vacuum full command on that table in PG Admin. What that does is actually free the space back up to the OS, and will also result in smaller Data Store backup sizes.
Edit: Just to add, I created an ArcGIS Idea several years ago about this.
