iMessage storage

Maybe phone B has iMessage backed up to the cloud?

There is backing up messages nightly using iCloud backup (backs up other apps as well), and there is iCloud for Messages (syncing iCloud Messages & devices including deletions and adapting what’s stored on each device as needed).

Seems like B is using iCloud for Messages or is cleaning up pics, videos, gifs, and even messages or both (straight deleting or deleting after storing them in another app like Photos). If using iCloud for messages on the device the true storage for Messages will show in iCloud settings.

If no attachments had been deleted by either before turning on messages for iCloud and conversation storage the same when turning on iCloud (and then same as listed on iCloud after uploads) have there been phone upgrades during conversation? When I get a new phone and turn on syncing with Messages in iCloud for new device a lot of old attachments aren’t downloaded unless I request them for a specific person or scroll back that far in conversation. If both have gotten new devices during conversation one may have downloaded all attachments for conversation and other did not.

I have also read that iCloud Messages may work a little different than iCloud Photos in regards to syncing with iCloud and reducing storage space on device and that the device you first turn on syncing with iCloud for Messages will keep all existing & future messages/attachments on device even after it uploads all to iCloud. New devices afterwards act like I wrote earlier in that they do not download all old attachments unless requested/needed but do keep new/future messages/attachments.

This hasn’t been behavior that I have seen and have seen attachments received on device that I had to download again later on same device (seemingly acting like iCloud Photos), but I install iOS betas and usually reset all settings on device with regular release which may have triggered need to redownload previously received message attachments.

Has B sent significantly more pics and videos than A? I'm just wondering if the conversation's pics and videos for B are stored with their photos instead of iMessage. While A, being the receiver, has all of that content in their iMessage. So as the originator of the content it's on B's phone but just in a different location.