What files is that dialog talking about? Source files so the forum’s condition can be reproduced reliably? Uploaded ones? Would be nice and easy to specify.
I’ve just migrated the forum from MyBB and there are no attachments (the migration doesn’t support them). What causes the difference in size? Is the last backup incremental? Are linked images from remote URLs downloaded in the first one?
If you have lots of duplicated images, there are tools that identify and replace duplicated files with symbolic links on Linux, in theory it should work well (never tested).
If you have a forum with lots of reaction gifs it would be a good case for file deduplication, I think.
Be carefull if you try this, backup everything and etc
Yes, our letter avatars only live on our CDN. It does reduce storage.
That said, user uploaded avatars are “optimized” multiple times for various sizes so user uploaded avatars are effectively stored 6 times or so in different sizes in backups.