I'm using a Twenty Thirteen child theme.
All of my product images are uploaded in high quality (2000+ pixels wide). Then the various WooCommerce thumbnail sizes are all properly created upon upload. I have not modified the default sizes for WooCommerce.
The problem is that I only have four products on my main store page and so WooCommerce is taking the smallest thumbnails available (150 x 150) and stretching them up to 220 x 220. This results in very poor image quality!
There is a 300 x 300 size created by WooCommerce, so I don't understand why it's stretching 150 x 150 thumbnails when it should be shrinking down 300 x 300 thumbnails.