There are two choices for displaying an image after you resize a window:
Do Not Rescale sets an image display mode where the window size has no affect on the image size.
In this mode, window resizing affects how much of the image and blank window space is visible. If the entire image is visible, shrinking the window may obscure part of the image; growing the window may add blank window space.
To turn off Do Not Rescale mode, use the View menu item Fit To Window.
The default is Fit to Window.