Well, that’s not what I meant. If you have directories with torrents or VMs, du might report different size for those directories on the source and target disks. Then it might’ve meant that those are the culprits.

With just the source disk, you can check du -hsc dirname versus du -hsc --apparent-size dirname to check if the disk space used is much smaller than the ‘apparent size’, which would mean there are sparse files in the directory, i.e. not fully written to disk. rsync would copy those files to full ‘apparent size’.

As mentioned elsewhere, btrfs might also save space on the source disk by not writing duplicate files multiple times — but idk if du would report that, since it’s specific to btrfs internals.