Why hasn’t the size of my “Mail” folder reduced after upgrading to macOS High Sierra? During the WWDC 2017 keynote, it was mentioned Mail in macOS High Sierra would use up to 35% less storage space. An article on thenextweb.com also says: “After installing High Sierra, Mail will begin the arduous process of compressing every message it stores. When done, old messages will reportedly take up 35 percent less space on your computer.”

However, despite having upgraded and having had Mail running continuously for over a day, I’m not seeing much, if any, reduction in the size of the “Mail” folder (inside the “Library” folder) as reported by the Info window in Finder. The window shows a total file size of 6.75GB with 6.87GB used on disk. Within a Time Machine backup from right before the upgrade, it’s 6.7GB with 6.9GB used on disk.

Why am I not seeing any reduction in the size of the folder? Is there something I need to do to get Mail to compress the messages? Or is this feature not available on all Macs?
To not leave my own question unanswered: I haven't been able to find much information on this, so far I can only assume Mail's reduced storage space is one of the features that was announced but didn't make it into the final release of macOS High Sierra, or at least not in the initial 10.13.(0) version.