To give Windows 8 a smaller footprint, Microsoft came up with a way to store some of Windows' files in the recovery image, & use them from there. That had problems updating Windows, so with 10 they came up with a way to compress them in place -- called Compact, it's only recommended for devices using SSDs, since because they're faster you [hopefully] won't notice the slight slow down from using compressed files. It came to mind because after upgrading my tablet to the Anniversary Update version it was turned off.
The easiest way to turn it on is to right click on the Start Button, opening a Command Prompt as admin, & typing Compact /CompactOS:always and pressing Enter. You can also type Compact /? for a list of available options, like just compressing some types or groups of files.
The results of turning CompactOS on aren't Earth-shattering -- maybe 1-3GB -- but sometimes that's enough to matter.