OS X 10.10.3’s new Photos app will ask if you want to upgrade your iPhoto library when you first launch it.
After Photos imports an iPhoto library, through the Finder it will appear that the Photos library (~/Pictures/Photos Library) is slightly larger than the iPhoto library (~/Pictures/iPhoto Library). In reality the actual size of the Photos library initially is quite small, and the size being reported by the Finder is not true thanks to some sort of Apple trickery. According to Apple’s getting started with Photos site https://support.apple.com/en-us/HT204655… “If you upgrade an iPhoto or Aperture library, your Photos library will be stored in the same location as the original library”.
The take away is NOT to trash your iPhoto library after importing into Photos, because if you do so, you will LOSE YOUR PHOTOS in both iPhoto and Photos! Trashing your iPhoto library after importing into Photos may seem like a good idea for reclaiming space, especially to those who are technically included as it would appear the import process duplicates everything. Don’t believe the size of the Photos library that the Finder is reporting!