I upgraded to Yosemite and migrated my iPhoto library to the new Photos app. I'm happy with Photos and don't plan to go back to iPhoto.
This Apple support article makes it clear that the photos don't really exist in two places, but leaves it vague as to where they actually are.
I don't like having an iPhoto Library that my Photos library is dependent on, and I'm bothered by the weird and misleading sizes reported by the Finder.
How can I get rid of the iPhoto Library without damaging my Photos Library?
Best Answer
Once you are certain you are happy with Photos, & you are certain the Library has migrated successfully, you can delete the old iPhoto app & iPhoto Library if you wish.
The Library, as you mentioned, is 'virtual' as both are hard linked to the same files. Deleting either Library will not delete the pictures, only deleting both Libraries will do that.
The apparent size on disk is incorrect [or rather, somewhat misleading], as each Library will claim to be approximately the same size, yet they refer to the same files.
Also see - Does migrating to Apple's new Photos app actually double my storage requirements?