I've added a bug/feature request that seems to address this thread.
The feature I would love would be for each session folder in the backup folder to be gziped or tarball compressed. This way, archiving to tape would be MUCH more efficient. It would be great if both the Zimbra backup and restore features were 'compression' aware, so you could restore from a compressed session folder.
Here's the bug/feature I added. Please comment and vote if appropriate.
Bug 31748 – Backup Session Compression