On 14/02/2019 03.12, Jason wrote:
Every now and again, I run the built-in Disk Cleanup
program to get rid of cruft - much left behind by updates.
I did so the other day and there was an element in the
list of items to erase that I hadn't seen before. (It was
unchecked by default.) cleanup was offering to compress
the OS drive and produced an estimate of how much space
that would save.

Windows could compress NTFS disks since at least two decades ago, this
is not new. It can compress entire directories or separate files
transparently.

And before Windows, there were MsDOS tools that did similarly.

--
Cheers, Carlos.