Phantom photos storage

I’m trying to free up space in my poor 64GB iPhone SE. As per phone storage, the main culprits maxing out the phone are 37000 pics taking up 20GB of space. They are all backed up on iCloud. I started out purging the 10k photos I found in Recently Deleted folder (and waited the requisite 30 days). The folder now shows empty, and yet no change in used storage. After trying every optimizing technique with no measurable result, I finally deleted every single pictures from the phone and un- Synched it from the cloud. I got about 5 GB free, but it is still showing photos for 16GB!!! How is this possible? Where is the phantom data!?