Now for the first time, those requirements are changing. Microsoft's Minimum Hardware Requirements page has been updated ahead of the public rollout of the Windows 10 May 2019 Update. It now shows that operating system -- Version 1903 -- will require at least 32GB of storage space for both the 32-bit and 64-bit versions. That's double the previous threshold for the former, and 12GB greater for the latter.
It's likely that this change won't affect too many people, as we doubt that many users are that lacking for storage space given how capacious most hard drives and SSDs are these days (not to mention our ever-increasing reliance on cloud storage). However, it's something to keep in mind when upgrading to the May 2019 Update, along with the guidance on removing SD cards and USB drives when performing the update.
Microsoft doesn't give any indication on why the storage requirements have changed so drastically since the October 2018 Update, but some are speculating that it partially has to do with this newest release reserving 7GB of storage for updates. This "Reserved Space" is needed to hold temporary files that are needed to ensure that Windows Update can get its job done without throwing up an error message due to a lack of disk space.
“Since disk space has been set aside for this purpose, your device will function more reliably," said Microsoft when describing the feature.
For those that don't want to wait until next month to get the Windows 10 May 2019 Update, you can follow these steps to get the latest build early.