What that post does clarify is that Photos apparently has an optimization target: 10 percent of free disk space. Photos is definitely optimizing the size of its library, though it’s still not entirely clear to me whether it only does this when it’s running, or if there’s some background process that might do it all the time. Plumbing the depths of the Internet, I found this pretty great post on StackExchange that charts the size of the Photos library and a Mac’s disk usage. In fact, there’s no communication at all from the app about how it manages its own storage space.
There’s no button for me to press to put Photos in Austerity Mode, no interface to force it to slim down what it’s using. That’s a lot, especially when you’re supposed to render an HD video in Final Cut Pro, but you can’t because you’re out of disk space. This thought occurred to me when I was fishing a file out of my Pictures folder and noticed that my Photos library takes up 46GB of my precious iMac SSD storage space. That’s as it should be, but a few optional controls for the control freaks among us wouldn’t be the worst thing in the world.
In true Apple fashion, Photos protects the user from thinking about managing storage - everything happens automatically, with absolutely no intervention from the user. She only ever uses these photos for Facebook and things like that - maybe some of them might get printed as 5'x7' prints for a photo album or something.
The library is taking up tonnes of space and it's bogging down iPhoto and her old iMac. When Apple released Photos for macOS, the company chose a clever approach to reduce Photos' storage consumption, knowing that most users would be upgrading an iPhoto library. 1 It does this by syncing the entire library to iCloud Photo Library 2 and then dynamically loading and unloading photos as you use it.
One of the more interesting features of Photos for Mac is its ability to not store my entire photo library on my Mac’s drive.