Where is my free space disappearing to?

[Android Central Question]

I have an unrooted S4 on stock android 6. I am almost out of free space -- AGAIN. A month ago, I ran out of free space for my apps and the only way I could fix that was to reload the operating system and reinstall the apps. I went from no space to 4gb available space. Now, just a month later, the space is getting gobbled up again. Why? I added a few apps (and deleted most of them after trying them). Aside from that I just do updates as pushed by the play store. I tried reclaiming space by clearing the apps caches, wiping the cache partition, moving apps or parts of them to the SD card, deleting junk, etc. Still it seems like after every batch of updates, I lose hundreds of megabytes of free space. I suspect somehow back up must be happening in the system area that I cannot access. But that's just a guess. So, how can I reclaim my lost free space. I really don't want to reload the phone again, especially since that's seems to be a temporary fix.