I think I may have worked out why my iPhone 5s space steadily keeps depleting to 0 over time (until I start getting the ‘Storage Almost Full’ messages). Everyone’s problems and solutions to Apple issues are so individualistic though, so I’m not sure whether my explanation will be relevant for everyone. I first tried everything listed in these kind of threads, such as deleting WhatsApp data, to logging out and back into iTunes account, etc. Nothing worked.
On your iPhone If you go on ‘Settings’ and then on ‘App and iTunes Store’ you will find a section called ‘Automatic Downloads’ with options for Music, Apps, Books and Updates. If you switch these on, the iPhone will sync purchases made from these categories. I figured that these had been interfering with the space on my iPhone so I switched them all off (Note also that I have never used iCloud and have always been logged out of it, so I do not believe that is affecting things).
Here is the strange thing – even when these options were switched off, the iPhone still seemed to register that I had made purchases (in my case, music purchases). I know this because every time I purchased music on my iTunes PC, the iPhone Music app would start showing a ‘Purchased’ playlist. I regularly have always deleted this ‘Purchased’ playlist, so I know that it has only appeared when purchases have been made on my iTunes account (even though I am specifically requesting that they not be synced to the iPhone itself). Once the ‘Purchased’ playlist is deleted it stays deleted, until I buy more music on my iTunes account. Although the music I purchase does not appear on the iPhone itself, I think there is some sort of glitch happening whereby the iPhone space depletes according to new purchases made. I think this would also be the case for buying and updating Apps, Books, etc. It’s almost like a ‘ghost’ imprint of your iTunes activity affecting the storage capacity of the iPhone.
Thus the only way I have been able to stop my space size from shrinking is to make sure I do not make any new purchases on my iTunes account, or update any apps. Since doing this, the space on the phone has remained consistent and I haven’t had any ‘Storage Almost Full’ messages for many months. It’s obviously not very convenient, but it’s the best solution I have found so far.