@theluthier the pixel size is the pixel size of your monitor. When you have a window smaller than full screen, it has less pixels.
If you have a 4K monitor, one pixel is smaller than on a 1920×1080 monitor.
But if you only use a part of your monitor, you have less pixels (of the same size though).
Look at the Iris example above; a one pixel stroke on the small image would be broader than a one pixel stroke on the full screen image.
It’s tricky to explain, but once you have your eureka moment, it becomes so evident (like many things in CG).
