There are a few things that my cause this.
1. Most caches are cleared after installing a new OS. Caches may become quit big after a few months of usage, up to a few GB's.
2. Your hibernation file is deleted after installing a new OS. The hibernation file is used by OSX to store the contents of you memory when you put your mac to sleep for a function called "safe sleep". In case of a power outage your RAM is cleared so your mac will resume from hibernation instead of sleep. So after you put your mac to sleep for the first time, the entire contents of your memory is stored on your disk including the empty space. If you have 8gb of memory installed you will have a hibernation file of 8gb on your disk.
3. In Mountain Lion, Apple has updated Time Machine so it will make local backups while your not connected to you backup-disk. It will keep these backups on your disk until the disk becomes too full so they can become huge. The moment you connect your mac to your backup-disk it will write all these backups to your backup-disk so you won't lose any backup. Chances are you made sure you have a working backup on your backup-disk before you upgraded to mavericks and so your local backups have been cleared.