What is the file format of the photos on your iPhone? Have the photos been taken with the iPhone in the JPEG or HEIC format? Or have they been synced to the iPhone from a computer and are in a different format, like TIFF or RAW? A JPEG taken with the iPhone may have a file size up to 15MB (for example a large panoramic shot), but a high resolution TIFF or RAW may have a file size up to 100 MB. 50 high resolution TIFF might take up 5 GB of storage. If you are using photo editing extensions, the extensions may be storing additional high resolution edited versions saved by the external editor. Have you been using external editors? Are there photos stored by other applicationsß
Even if you are syncing the iPhone with iCloud Photos, the photos will take up local storage on your iPhone. The photos and videos are stored in iCloud, but are synced with the iPhone and mirrored on the iPhone. The local library on the iPhone will always be identical to the iCloud Photos Library. If you enable "Optimise iPhone Storage", Photos will remove some of the high resolution originals and replace them by smaller, optimised versions, but only, if you are running low on storage and need storage for other applications. "Optimise" will not remove downloaded photos needlessly but keep the most recently downloaded photos readyfor use, to avoid network traffic.