This could be filling up your phone without you knowing.
Try this:

Screen Record for 10 seconds from your phone (try a You Tube video .. doesn't have to be full screen).
The file size will be about 20MB,

Open it and select the default "Video Player" app (Purple icon with white triangular button).
If you didn't see that and it opens in another app, reset that so that you get the choice (not explained in this post).
Pause the video a few second in, and hit the three dots, top right and chose 'Editor'.
Video will open in the Editor.

Do some edits:
- Severely Crop the video smaller
- Trim it to 2seconds
Then press Save top right.
Check the video size in "My Files" in the "DCIM>Screen recordings" folder.
It will now be say 2MB is size . . . as expected, smaller.

Reopen the edited file in Editor, and now press 'Revert' in the top right and then press "Revert" and then "Save".
Re-look at the file size in "My Files", and you will see it has reverted back to its original file and size.

Where is the original video being stored when you do the editing?
Clearly on the device still, otherwise you wouldn't be able to "revert".

If you MOVE the smaller edited file off the phone and move it to a PC,
the PC sees a small file. The truly edited version.
The PC knows NOTHING about the original larger file on the phone or the editing on the phone that took place.

What happens if you removed the file from the phone to a PC, rename the file and put the file back on the DEVICE (phone)?
You would expect that the phone would see it as a fresh new small file.
But no, when you go to the "Editor" again, even though it now has a new filename,
it somehow allows the 'Revert' option again, which means the original, larger, unedited file was on your phone but hidden.

Even if you make copies of this smaller edited file either on the computer or on the phone,
EACH copy, once back on the phone and into edit mode KNOWS that the full unedited version exists as the revert option is available and works.
It doesn't even matter if you move the files back into another folder on the phone.
It somehow knows that full hidden file is somewhere on the phone.

If you had a 1000 large files that were edited to very small files, and each one only SAVED (not "Save as copy")
and then you move the small edited files off the phone to a PC,
the large unedited files are on the phone somewhere taking up space. . .this can't be right