Mavericks frees >12GB of SSD
ETA: while the guesses below all stand, here’s a better one from Marceil Weiher:

@rentzsch Have you checked with &quot;df&quot; vs. looking in Finder?  I think 10.9 subtracts the space for mobile Time Machine from used in latter.
Marcel Weiher (@mpweiher) on Twitter

I’ve done some poking around on the machines I have lying around, and I think this is plausible and probably correct.
If you use Time Machine, that’s probably your answer.

Without seeing your system, it’s hard to know, but here are some suggestions:

The Mavericks installer is ~5GB in size, but it gets deleted once you’ve finished upgrading. When did you take the first reading? Perhaps you’ve lost the 5GB installer file (which replaces a lot of existing system files, so it doesn’t add 5GB to the system files), which would contribute to the difference.

Caches (temporary files) often get purged during an OS update. I have ~3GB of caches on my Mac, which would significantly free up space if that happened. Perhaps you had a lot of cache files to begin with?

Do you use Time Machine? Starting with Mountain Lion, Apple started keeping backups on your local disk, and writing them to an external drive when you plug it in. Assuming you plugged in your backup drive before you did the update1, it might just have purged all those local backup files and saved some space.


1 You did, right?