Not the normal thing I would post, but I think this is significant. I think you may be seeing that more and more as the games get larger and larger.
For previous expansions, the minimum requirements list 70 GB on a 7200 RPM HDD (hard disk drive), the recommended specs list 70 GB available on an SSD, as an example. However, Shadowlands will require 100 GB of SSD storage for both minimum and recommended specs.
Forr, check out this CSS Engineering story.