Something I forgot to mention-
apps such as Facebook, Facebook Messenger, Telegram, and many others like to store files in their own folders.
For instance, say you see a funny meme on Messenger, and you select 'Save To Device'.
The app has already made a folder just for Messenger, and the media from that app will be stored in there.
Depending on whatever Gallery app you are using, the placement of the file may or may not be immediately apparent.
Also, some apps, like Telegram, have the tendency to download every picture and video that you see while using the app.
Once again, this media would be stored in a folder that you may not be looking for immediately.
The worst offenders are apps that hide such data inside of their Android folders, andd the worst of the very worst keep files inside their root files- where you will have no access to them.
For a long while my Galery app was growing to almost 2GB in size every week- making it not only the single largest app on the device, but also about the same size as the system itself on one of my devices.
This was so infuriating!
It turns out that there is another 'cache' in the app, this one is not accessible by the user.
I had a few arguements with the developer over this, as he was trying to tell me that my device was just too low on resources.
I had to point out that although I agreed that my device is pathetic, it meets a standard that the vast majority of worldwide users are using {at the time, about 80% of devices were at this standard}.
Anyway, what was being stored here?
Evidently it was a 'copy' of every thumbnail from every picture and video, and maybe even some whole files, from media that I had deleted.