I have often wondered what is the difference between these two buttons on the Image VIewer menu bar? Superficially, the two buttons do the same job.
It is "Fit to Page by shrinking" vs. "Fit to Page by growing".
If the image is larger than the window, they both do the same thing and reduce the image to fit it into the window.
The difference is when the image is smaller than the window. Fit To Page leaves the image alone and displays it at 100% size with a border around it. Grow To Page enlarges it to make it fill the whole window.
Or, to quote the manual:
Fit to page: Push F . Large images are reduced if too large for the window, but images are never enlarged. (Aspect ratio is always preserved.)
Grow to page: Push G on the main keyboard. Large images are reduced to fit in the window; small images are enlarged to fill the whole window. (Aspect ratio is always preserved.)