Microsoft revealed on its Hardware Dev Center that both the 32-bit and 64-bit versions of the Windows 10 May 2019 Update will require 32GB of available storage, a marked increase from previous versions of Windows 10.
Just look at the Windows 10 October 2019 Update and its storage requirements. The 32-bit version of the operating system requires 16GB of storage; the 64-bit version requires 20GB. It's not clear why the May 2019 release will double the requirements of the 32-bit version of Windows 10 and raise the 64-bit version's requirements by 12GB.
On MSFT noted that most devices these days have far more than 32GB of storage--even some high-end smartphones have started to offer capacities of up to 1TB. We suspect that pretty much everyone who has less than 32GB of storage on their system was probably either looking to upgrade their hardware as soon as possible anyway or already decided they don't mind using outdated technology as long as it works for them.
But taking up an extra 12-16GB of storage also isn't a small change. Some people buy the smallest possible drives to run Windows 10 so they can boot quickly without having to break the bank on higher capacity products. Others simply don't have a lot of space to begin with, and taking up a dozen extra gigabytes of space might push already strained setups past their limits. They can install this update, sure, but they might not like it.
Odds are good that the number of people who can't (or won't want to) install the Windows 10 May 2019 Update is pretty small. Just don't be surprised when the update rolls out--supposedly at the end of May--and requires more space than before. Microsoft didn't say why, but we assume it's not just because it secretly wants to push Windows users towards higher capacity drives.