After some digging the plugin is using woocommerce_single to get the image size for the URL of the image for Pinterest.
So I guess there is two fixes for this:
1) Increase the size you are using for woocommerce single product images so a larger resolution image is used for Pinterest. However this might not be ideal for everyone.
2) In my case I would like the full image for Pinterest which I also use on the single product page with zoom (size not defined with woocommerce_single in my theme ).
So my fix:
Edit Line 307 in:
/wp-content/plugins/pinterest-for-woocommerce/src/ProductsXmlFeed.php
Change:
$image = wp_get_attachment_image_src( $image_id, ‘woocommerce_single’ );
To:
$image = wp_get_attachment_image_src( $image_id, ‘full’ );
This will generate the full image URL in the XML file.
I am not entirely sure how to override this within the theme functions and not modifying the plugin directly, alternately the developer could allow us to decide in the settings menu or just use the full size image by default.