There are two choices for displaying an image after you resize a window:
Fit To Window sets an image display mode based on the window size. In this mode, when the window is resized, the image is resized. The image size matches the window size apart from window space needed to maintain the image's height-to-width ratio.
To turn off Fit To Window mode, use the View menu item Do Not Rescale.
The default is Fit to Window.