What is the maximum/optimized size of the OSX photos.app library? I was using aperture with referenced images. The aperture library is about 20gig, the folder containing the referenced images is 200gig. I exported versions of a selection of my pictures (6000) and imported those into photos.app for a trial run. The settings (checked) in photos.app are: iCloud Photo Library, Optimized Mac Storage, My Photo Stream, iCloud Photo Sharing.



Right now my Photos Library has a size of 23gig. Which is used mainly for Masters as seen below: 



When checking my iCloud-Setting (in System Preferences) the size of photos in the cloud is 16gig. 



I use a MacBookAir(late2010) right now with 128gig of storage. So I was hoping photos.app would upload all the pictures in full-res to the cloud and keep low-res pictures locally. Right now it seems the local copy is even larger. I had my mac book on wifi (and connected to power) for some days now (only 78 pictures left for upload).

So my questions are:


What is the target size Photos.app is optimizing for?
Is there any way to change/adapt this target size?
So after some time (learning python on how to monitor and plot library size and disk usage) this is what I have observed regarding my first question. As written by @will-mallard and @bmike the optimizer targes for percentage free disk space. As shown below: the size of the picture library increases and decreases together with my disk usage. During those three days I did not use my mac book air for work. I only added pictures two times.


Over a longer time scale, one can see the reduction of picture library disc space over time while I was working on my macbook.


My followup question is now:
What triggers the optimization? At first I thought it is some kind of free space threshold. But as it is easily seen in the last plot, something at the 24th May triggered a reduction of library space. I am not sure what that was.