This may have to do with the image dimensions you’ve set in the Dashboard -> Settings menu for the different possibilities – thumbnail, large, etc. I don’t know how Woo sets the image sizes for product pages, but it is not uncommon to use thumbnail sizes for mobile devices. That may be a Woo setting, or you may need to up the dimensions you have for thumbnail images, then use a plugin to resize all your images based on the new sizes you set. (WordPress usually saves the original full size image, then creates the other sizes on the fly when originally uploaded – you need to use a image resize plugin to get WP to reset all the images to the new sizes.)