Why does the mobile version of the theme (Xstore) uses the original (full sized) product images and not the thumbnails?
It only happens when you show the shop on a mobile, not the mobile view on the desktop. It happens not only in my webstore but also in your demo theme.
You can also view it on Google Pagespeed Insights and see that the mobile version uses the full images.
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The size of the images that are used for the products depends on your settings in WooCommerce > Settings > Products > Display > Image Sizes.
We use catalog image size for the products grid http://prntscr.com/en3dfz
But Google Speed tool tells you that you can optimize original image because at present it’s too large. If you optimize original image and then regenerates thumbnails all the image sizes (catalog images, thumbnails) also will be optimized. Please, download optimized images and use them for your products http://prntscr.com/en3t5v
No, you see that Google Pagespeed shows you notice about these products and recommends you to change the images. It even generates images. You can download the archive http://prntscr.com/en3t5v then edit products and replace images by images from the archive. After that test the site again.
Mobile version does not use real images. Google speed tool defines that catalog images can be optimized, but catalog image is autogenerated image, WP uses real images and autogenerates catalog images and thumbnails. So, only if you optimize the original image you’ll get the optimized catalog and thumbnail images. That’s why Google speed tool shows you these notices and offers you real images.