rsync from external vfat disk to local ext4 yields VASTLY different sizes

I found an older external hd I want to reuse for something else. I was doing an rsync of it to a NAS I run over the network but it was taking ages. So I decided to rsync to my local drive first (SSD) and do the final backup to NAS later. I ran rsync -avvz --progress /media/ubuntu/9AB4-7DB9/ubuntu/ bak. This seems to have terminated fine. But when I compare the two dirs for their sizes, they are VASTLY different.

du -kh bak
29G	bak

du -kh /media/ubuntu/9AB4-7DB9/ubuntu/
56G	/media/ubuntu/9AB4-7DB9/ubuntu/

How is this possible? I first assumed that the vfat file system might be to blame - but to this extent? I can't believe it to be nearly doubling the size I also thought it could be the -z compress option of rsync, but that should only compress during transfer as I understand: -z, --compress compress file data during the transfer Any ideas? I am baffled, and just want to make sure my backup was complete. Thanks,