Image view size different depending on program

[gregl7]

I've been using Irfanview for a few years now, and it wasn't until today that I noticed something I'm trying to figure out. I can take the same image and open it in different programs and it will show as different sizes on my screen, even with the view/zoom size set to 100%. When I open an image in Irfanview and set the zoom size to 100%, or if I open it in a program like Microsoft Paint, it will show the image as the same size in both programs. If, however, I open up the image in another program like a web browser or Microsoft's built-in "Photos" program in Windows 10, the image will appear as the same (larger) size in both programs. Does anyone know why that is? I would think viewing an image at its 100% size should be the same in every program you view it in.

[Bhikkhu Pesala]

Other programs may zoom in, e.g. to fit the image to the view port. 100% means 1 pixel in the image = 1 pixel on the screen.