Hello,
The Product Images aren’t influenced or affected by the Theme. It is controlled at the WooCommerce end through the available option that you already know under Appearance > Customize. Now, there are two ways available right now for it, i.e. either you can upload resized Images for the Products according to the standard WooCommerce Product Images size which is around 800×800 pixels or if the number of images is large enough and you don’t want to do that then a CSS solution could be tried off in this regard. If it works out for you:
.product-grid-item :is(.product-image-link,.hover-img) img {
object-fit: contain !important;
width: 20em !important;
height: 20em !important;
}
So, we have applied the above Custom CSS on your Site in this regard. Before applying the CSS, we remove the size 500 defined for the Thumbnail Images so that the CSS can work correctly, just to mention you.
Please check back your Site after completely clearing the browser and server cache.
Best Regards