iPhone 7 Plus running iOS 12.4.1.
Device showed the following iMessage statistics under iPhone Storage:
I took encrypted backup using iTunes on Windows, then restored that backup (also to 12.4.1). Waited 24 hours following the restore for everything to sync, iCloud messages to download if necessary (Messages in iCloud was set to ON), etc. But same screen shows vastly different numbers now:
Things like this about the iPhone – there you have zero visibility into what is actually happening freak me out. What would account for the substantially lower storage numbers afterwards?
UPDATE: I am convinced that all of the photos and videos reflected in the "before" screenshot are still present on the device (despite Messages in iCloud being on), yet for some reason the Messages storage page simply isn't reflecting the usage for some reason. The reason I believe that the media is still present on the device is because when I back up the iPhone via iTunes and then use any number of 3rd party tools to inspect the backup, all of the photos and videos are there.
Best Answer
I updated iOS* and the screens shown in the screenshots above showed the results I expected; that is, they showed storage amounts consistent with those prior to my restore.
I suspect that the upgrade forced the statistics to be updated.
*The upgrade was to iOS 13.1 but my gut feeling is that any upgrade would have "resolved" the problem.