I had a 250GB hard drive in my Macbook Pro with ~60GB available space. It started making some noise so I backed up and brought the machine to the Apple store. They replaced it, the screen and 2 fans and gave me a fresh install of the OS. I did a time machine back-up and now I have 110GB of free space. I'm not missing anything that I've noticed so it seems weird that I gained ~50GB from a fresh install/back-up... 