For me, the deduping and compression saves a lot of storage. My mail backup (17 backups covering the last 6 months) is originally 837GB, compressed to 312GB and dedupe'd to 19GB. Same with Postgres - 25GB to 7GB to 900MB.

You could probably use rsync's hard linking to save space on the mail backup but I'm not sure you'd get it as small without faffing about.