This is an example of how this image, with original size of 400px X 484px, will not grow larger than it's original size if the maxAtOrigImageSize option is set to true.
Try resizing your browser to smaller than the image. It will scale the image smaller, but no larger than the original image size.
Here is the code that is used to build this page:
<img src="images/maxatsize_tester.jpg" alt="Tester" class="maxAtOrigImageSize" />