I've had multiple relatives (ahem, samsung) phones somehow waste data...
I'm talking 32gig phone, with 8gb 'system', 20gb "other", and a few apps. I can delete all the apps, app updates (because preinstalled apps are installed twice, once in the system partition and then a new version in the data partition), and 20gb is still missing. I can mount the phone, and find 5mb of random files, zero photos (i removed them earlier), and still, 20gb missing. I have a theory, that samsung updates leave some trash behind somewhere on the system partition and never clean up after themselves, because after a factory reset, and full upgrade, the space is back, even with all the apps and updates.
I don't want to root those phones, becase they're not mine, and this brings in more issues (eg. eternal support), and factoy reset seems to be the only way to "fix" them.
Semi-related: Older Android versions (up to 4.1.2 or so) used to have a bug whereby old native libraries weren't deleted when updating an app. I remember Skype being a particular offender, since their main native library included a version number in the file name, so with every update a new version of a multi-megabyte .so would be plonked onto your phone, but with all the previous versions of the library being left behind on your storage, eventually taking up a considerable amount of space after a couple of updates.
Unless you were rooted, the only workaround to free up that space as an end user was completely uninstalling and then reinstalling the app. I tried contacting their support about it, but it was of course useless in trying to get them to understand the problem…
Google in turn used to ship a number of 0-byte .so-files with their apps in order to circumvent the problem.
I had the same problem on my iPad. The “Other” category was taking up several GB and the only way to get the space back was to wipe it and restore from backup.
It’s infuriating that Apple allowed it to happen in the first place. It’s yet another slow-motion quality degradation like the battery-induced downclocking a couple years ago.
Did you try (temporarily) off iCloud in photos? On one iphone, 85G was Other (<3G remaining free) until we turned off the iCloud stuff in Settings→Photos. Seems like some bug, since the iCloud account was capped at 50G and the feature was supposed to store "smaller versions" of the full size stuff in the cloud …
I'm talking 32gig phone, with 8gb 'system', 20gb "other", and a few apps. I can delete all the apps, app updates (because preinstalled apps are installed twice, once in the system partition and then a new version in the data partition), and 20gb is still missing. I can mount the phone, and find 5mb of random files, zero photos (i removed them earlier), and still, 20gb missing. I have a theory, that samsung updates leave some trash behind somewhere on the system partition and never clean up after themselves, because after a factory reset, and full upgrade, the space is back, even with all the apps and updates.
I don't want to root those phones, becase they're not mine, and this brings in more issues (eg. eternal support), and factoy reset seems to be the only way to "fix" them.