Well, the WooCommerce settings are kicking in. They are 300x300: http://cld.wthms.co/ECWn
But, the width of the image is determined by its container - and you've set that to be 20% of its parent. So the browser is displaying it at 143px x 143px.
You can either 1) reduce the number of images per row or 2) reduce the size of the catalog thumbnails in WC settings.