Since installing the Yosemite beta and trying out the new Photos application I've noticed my hard drive space decreasing and thought to make a note in case anyone else wondered about this. When moving to Photos the original library is hard linked meaning you get a second copy of the original library for no extra space taken up. But as you use the new application to edit photos, new files must be created to accommodate the differences. Since my library is large and I can't fit 2 copies on my SSD I will soon have to delete my old (backed up) library. With the hard links this will not delete all the photos but instead the old versions of photos I have since modified.