The latest Windows 10 Insider build, number 18312, introduces a new feature wherein the operating system reserves a big old chunk of disk space, effectively expanding its on-disk footprint by another 7GB.
The storage reservation is to ensure that certain critical operations—most significantly, installing feature updates—always have enough free space available. Windows requires substantial extra disk space both during the installation of each feature update (as it unpacks all the files) and afterward (as the previous version of Windows is kept untouched, so that you can roll back if necessary). Lack of free space is one of the more common reasons for updates failing to install, so Microsoft is setting space available on a long-term basis, allowing those periodic updates to be sure they have what they need.
The exact amount reserved will depend on the optional features and language packs installed, but about 7GB will be typical. The reserved space isn’t entirely lost during normal usage, with certain temporary files having their disk usage charged against reserved space rather than free space. Being temporary, such files can be safely discarded each time an update is available to reinstate the full reserved capacity.