I came from OSX Mavericks, and since the update to Yosemite I gained 20+ Gigabytes of space. I would usually expect to lose space. Did the update delete some files that I am not aware of?
As you know OS makes a lot files during its uses.
Many of those are hidden files or Cache or Temp files but all related to the specific OS.
When new OS is installed it takes over those (deletes most of them) and starts making its own ones.
For example the Terminal command
will show you the cached files/folders, but this will only show the non hidden ones. I have some 3 Gig of stuff in there.
OS will hide system files and ones it creates in due process to protect them, but they can be made visible if one wants to do that.
I can not say what specific files new OS install will remove but they must be specific to a previous OS and no longer needed by new OS since it will create its own versions of them.
Could it be that installing Yosemite "purged" backups prepared when you were still on Mavericks? It did that with my machine at least.
It could be that as part of the upgrade process, backups prepared on the previous OS version are pruned.
Apple's KB on the minimum requirements for OS X Mavericks states:
Your Mac also needs:
- OS X Mountain Lion, Lion, or Snow Leopard v10.6.8 already installed
- 2 GB or more of memory
- 8 GB or more of available space
The Wikipedia page for OS X Yosemite indicates the systems need approximately the same amount of free space:
All Macintosh products capable of running OS X Mavericks are supported by Yosemite; as with Mavericks, 2 GB of RAM, 8 GB of available storage, and OS X 10.6.8 (Snow Leopard) or later are required. However, the system requirements in order to take full advantage of the features of Handoff include a Mac with Bluetooth LE (Bluetooth 4.0), and other currently unknown factors.
So unless you're noticing some pretty big files missing, I don't think difference in OS size can account for the freed-up space. Given that your system is behaving normally, I'd say Mavericks TM backups were deleted as part of the Yosemite installation process.