Reserved storage starts by reserving about 7 GB of hard disc space and therefore the amount of space reserved will vary over time supported the device. The reserved space are often reduced by removing unused optional features and languages, such as:
Installed optional features: Many optional features are available for Windows. These could also be pre-installed, acquired on demand by the system, or installed manually by you. When an optional feature is installed, Windows will increase the quantity of reserved storage to make sure there’s space to take care of the feature on your device when updates are installed. You can see which features are installed on your device by getting to Settings > Apps > Apps & features > Manage optional features. You can reduce the quantity of space required for reserved storage on your device by uninstalling optional features you’re not using.
Installed languages: Windows is localized in many languages. Although most organizations only use one language at a time, some users may switch between two or more languages. When additional languages are installed, Windows will increase the amount of reserved storage to ensure there is space to maintain these languages when updates are installed. You can see which languages are installed on your device by getting to Settings > Time & Language > Language. You can reduce the quantity of space required for reserved storage on your device by uninstalling languages you’re not using.
When enabled, reserved storage will instantly reserve its full allotment of disc space. However, on disk-space-constrained devices, enabling reserved storage will leave the user space and can only take the minimum—which is 2% of system volume capacity or 3GB of disc space, whichever is lower to make sure that the device is functional and accessible to the user for further operations.
Reserved storage will grow back to its original allocated size as space becomes available, like when old Windows installations are removed or then Storage Sense cleanup tasks are conducted.