Traditionally, Microsoft would use the release of a new operating system to bump the minimum hardware requirements that the software needs. With Windows 10 being the “last” version of Windows, Microsoft is using the major updates to bump specs. The May 2019 update, version 1903, takes the opportunity to do just this.
Previously, 32-bit Windows had a minimum storage requirement of 16GB, and 64-bit Windows needed 20GB. Both of these were extremely tight, leaving little breathing room for actual software, but technically this was enough space for everything to work. That minimum has now been bumped up: it’s 32GB for both 32- and 64-bit versions of Windows.
Part of this growth may be due to a new behavior that Microsoft is introducing with version 1903. To ensure that future updates install without difficulty, 7GB of disk space are permanently reserved for the install process. While this will avoid out-of-disk errors when updating, it represents a substantial reduction in usable space on these low-storage systems.