Free space after Yosemite install?
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

open $TMPDIR../C/


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.