Main image resizing to width of thumbnails

I’m having trouble getting the slideshow to maximise to the full width of the page. I have selected full from the image size menu, and as the page loads you can see that image starts off large but rapidly shrinks to the width of the thumbnail area – if I add or remove images to the slideshow the main image gets larger or smaller.

I’ve checked and the surrounding div in my theme (a child of twenty-twelve) is the full width of the page – inspecting the page shows that the div portfolio-slideshow0 is being given a fixed size but I can’t find where that is being set in the code to try and fix it.

Any help would be greatly appreciated as otherwise this plugin is exactly what I’m after…

You can see the problem here:

http://www.thetreehouse.org.uk/corfu/properties/agnos-ridge-house

Thanks!

http://wordpress.org/plugins/portfolio-slideshow/