PMiles:
Hard links are used in Photos to prevent duplication of media. You can verify that yourself by exporting some of your library to a small USB drive, enough to more than half fill the drive, then using Option Open to create a Photos library from the exported one. You'll note everything fits on the small drive. The use of hard links (multiple names for the same file) is what allows Time Machine to keep many many dates worth of backups without duplicating files that have not changed between dates.
Also, here's a quote from David Pogues's review: https://www.yahoo.com/tech/everything-worth-knowing-about-switching-to-os-x-1101 29491789.html
"What’s wild is that Photos accesses your existing photo library. It doesn’t convert the original photos into some new format. You’ll be able to switch back and forth among the three programs — iPhoto, Aperture, Photos — without having to duplicate any files or use up any more disk space.
There is one big catch, however: Your library splits at the moment you install Photos.
After you first run Photos, when you edit, add, or delete any pictures, those changes show up only in the one program you’re using at the time. New pictures you add to Photos appear only in Photos; new pictures you add to iPhoto appear only in iPhoto, and so on. (You can easily export/import them if necessary.) Same thing with edits you make: They’re saved only in the program where you make those changes."