Ah I see, yes the image is set to a particular size in fancybox, where as on the post it’s set to fill the width of the frame it’s set within. You can change this using css, to make the fancybox image larger, but remember this new size will apply to all images.

#fancybox-content {
border-width: 10px;
height: 500px;
width: 500px;