I realized yesterday that I had no disk space left on my iPhone 5 after I got spammed with iOS notifications that my apps could not be updated (due to lack of disk space). This morning I woke up to about 350 MB of free space - and I didn't do anything.
I don't know what the exact numbers were for usage (Settings --> General --> Storage & iCloud Usage --> Manage Storage), so I don't know what happened. As well as gaining free space, about 10 apps were updated.
My best guess is that there is some internal routine to compress images, since at full resolution they take up a fair amount of space, but even minimal lossy compression can save megabytes per photo without a noticeable loss in quality.
Is there such a documented process, or did something else happen? In any case, I wish I would have screenshotted the storage usage before this happened...