My wife has an iPhone and I have enabled the iCloud sync and storage optimization so I expected that photos were to be deleted on the iPhone if the free space would become scarce.
However, several GB of storage are still in use by photos after the memory consumption reached a critical threshold. All photos and videos are synced to iCloud.
Photos optimizes storage by removing the full-resolution files. If you have a very large photo library, it could still take a few gigabytes of storage to store the thumbnails for browsing your library.
There is no way around this, however it should not exceed a fraction of the smallest storage iPhones that can run the latest versions of iOS, even for very large libraries.
If you're seeing larger usage (20+GB) then either there's not enough pressure to purge storage, or perhaps it's working on it and it may take a moment.
Of course, everything is very simple there I don't know, maybe you have already done this, but first of all you need to go to iCloud.com, namely to your account and select the Photos folder, and just select what you don't need and delete it and that's it.
