When switching computers and migrating data, I found that photos in iCloud took up 70 GB, but on my Mac, they took up 140 GB. Apple system bug... I checked on reddit, and the solution is to repair the photo library, with the steps as follows: If someone finds this method years later like I did, I successfully reduced my total photo size from iCloud (781GB) from 21% (165GB) to 9% (70GB). Apple states that the "Optimize Mac Storage" option should use about 10% of the total size, so this is pretty much the best result. Open Photos -> Settings -> Enable iCloud Photos -> Optimize Mac Storage Close Photos (make sure it is completely closed) Open Finder Open the Applications folder Find Photos Hold down cmd+option and double-click to open Photos You will see a repair interface for Photos, click "Repair" Let it run, and your photo storage should shrink to closer to 10% of the iCloud photo size.