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?