My screen resolution is 1920x1080 pixels. So, if I'm viewing an image that "fits" those dimensions, if it is "horizontally" oriented, I can see the image at its "native" resolution. For example, if I size an image for displaying outside of POTN, I can size it to 1080 pixels at the highest, meaning that a horizontal image could be 1620 pixels wide, 1080 pixels tall. So, if my software is designed to display the image at "full size", that's what I would see on my screen.
But if I rotate it, of course, the viewing software would either show me just 1080 pixels of the image in height (if viewing at "full size") or it would "fit to screen", meaning to shrink the 1620 pixels down to 1080 for viewing.
Typically, viewing software gives you the option.