Problem
ISC uses the small “standard” image for the Open Graph image tag with
<meta property="og:image" content="https://www.moddiy.com/product_images/w/874/TB2AriHq3MPMeJjy1XbXXcwxVXa_%21%2165511238__63402_std.jpg" />
Solution
In /public_html/includes/classes/class.product.php
Find
$this->_opengraph_image = $productImage->getResizedUrl(ISC_PRODUCT_IMAGE_SIZE_STANDARD, true);
Replace
$this->_opengraph_image = $productImage->getResizedUrl(ISC_PRODUCT_IMAGE_SIZE_ZOOM, true);
Result
<meta property="og:image" content="https://www.moddiy.com/product_images/f/021/TB2AriHq3MPMeJjy1XbXXcwxVXa_%21%2165511238__07684_zoom.jpg" />