Product sizes vary on the shop page

Here’s the problem:
https://dl.dropbox.com/u/5581800/Screen%20Shot%202013-01-06%20at%208.22.39%20PM.png

All the images are sized to 400×400 but yet, randomly, images show smaller on the shop page. http://www.greyley.com/shop/

http://wordpress.org/extend/plugins/woocommerce/