icloud photo library space management

So I have it setup that my device manages the space so it is setup to "optimise iphone storage"

When I tap a photo i can see it downloading - is it downloading the full res version at this point in time and if so - does that mean the more photos i look at the more it will download and as a result take up more and more of my storage the more i browse? Or does it download full res just for the time i am looking at it then say an hour later it has removed the full res and gone bac to the low res version stored on the device?