IF am understanding, would agree with Emma ... doubles.
However, what does "in a Unix-like way" mean? Do you mean like rsync? IF you are thinking in terms of rsync, the answer would be no ... backups are an archive - tar.gz and the only way to get rsync like behavior with those is (am thinking ... have never done it) to open the tar.gz and replace only those files that have changed in the archive and then re-archive it. If that could be done then the backups would/could either shrink or grow (most grow, I would imagine). While some files in a backup are compressed and thus save space, digital type files ... like videos ... won't get much compression. Plus, the un-archiving would/might require most of the free space ... depending.
I have a preference (due to experiences with various systems that developed problems with the new file system in vr 2->) .... autobackups go to a designated directory where I can easily see their filename and size + the number of backups. Autobackups to the file system presents a potential issue with DB matching what's physically present in filedir (sea). That's not supposed to happen but it has! Autobackups also might fail due to the shear size of the course ... digital photography .... video editing ... etc. Help with one system that has such courses and checked on it the other day ... one course began this year as a 50 Gig backup ... it's nearing semester now and the backup is 90+ Gig and by the break will be over 100 Gig (have never really tried to restore such a course ... and am imagining there might be issues attempting that via GUI interface.
Some might have 'unlimited' space (if there is such a thing), but many don't. The very last thing one needs to happen is for the drive/partition upon which the DB files reside running out of disk space due to run away auto backups that one can't find without a DB in the 'sea of files'.
My 2 cents, of course!
'spirit of sharing', Ken