Your gallery will remain the same size on your page whether or not the thumbnails are displayed.
When the thumbnails are displayed, there is little room left within your gallery to display the main image (and it is displayed as large as it can be with with space available).
The only way to display the image at its larger size when the thumbnails are displayed would be to change the size of the gallery when the thumbnail button is clicked (to give your gallery more height to display both the thumbnails and the main image).
This can be achieved by using the Juicebox-Pro API, specifically the setGallerySize() method and the onShowThumbs() event.
To see an example of this in action, create a sample gallery with JuiceboxBuilder-Pro (using default values) and replace the gallery's embedding code (in the 'index.html' file) with the following:
<!--START JUICEBOX EMBED-->
<script src="jbcore/juicebox.js"></script>
<script>
var jb = new juicebox({
containerId: 'juicebox-container',
galleryHeight: '600',
galleryWidth: '600',
showThumbsOnLoad: 'TRUE'
});
jb.onInitComplete = function() {
if (jb.getScreenMode() === 'LARGE') {
jb.onShowThumbs = function(showing) {
if (showing) {
jb.setGallerySize(600, 600);
} else
jb.setGallerySize(600, 400);
}
}
};
</script>
<div id="juicebox-container"></div>
<!--END JUICEBOX EMBED-->
Your gallery has dimensions of 100% x 100%. This means that it will fill its parent container no matter what its size is. However, in order for Juicebox to determine what its actual dimensions should be, please be sure to explicitly set dimensions for the gallery's parent container using CSS (otherwise Juicebox will not know what the 100% should refer to).
When using percentage heights, please also see this note regarding Using Percentage Heights]Using Percentage Heights
If using the API solution above, this will not apply as setGallerySize() accepts only absolute pixel values (not percentages).
Also, your web page has HTML errors which may be causing problems.
You can validate your web page with the W3C Markup Validation Service and then fix the errors reported.
Once the code on your web page validates, your web page should be rendered with greater predictability and consistency across different browsers.
You also have a very high thumbSelectedFrameWidth which makes your selected thumbnails look a little strange. I would recommend reducing this value from 45 to something less than or equal to half the height/width of your thumbnails.