What is the maximum/optimized size of the OSX photos.app library?
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.