If you have a Samsung phone and you don't understand where you are loosing most of your space, you can take a look to the CloudAgent Application.
The CloudAgent app is the one behind the Cloud menu inside the settings one
You can check and disable this settings going into Settings -> Cloud menu and then selecting Pictures and Videos menus to check if the cache is activated and to disable it.
When you disable it, all local (cached) files are automatically deleted (but Dropbox uploaded photos and local taken photos are not impacted!!)
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