In the backend of your theme , please go to WooCommerce > Settings > Catalog. On the bottom of the page you should have 300x300 with the hard crop checked for Catalog Images.
Also please look in backend Settings > Media and make sure the checkbox on that page - (Crop thumbnail to exact dimensions (normally thumbnails are proportional)) - is checked.
You will have to regenerate your thumbnails before the difference can be noticed, so please install this plugin to automatically get new thumbnails.. http://wordpress.org/extend/plugins/regenerate-thumbnails/
This is a good post explaining the concepts further http://10weekwriterschallenge.com/?p=52