Jasper de Groot

The images on the provided sample page are inserted at full size. See the width attribute and the class names of the image elements. The sizes attribute is based on this.
For images that you insert in the post with the editor, use image size large instead. This will be constrained to the content width ($content_width) that should be defined by the theme. For featured images that are included by theme it depends on what code is used to insert the image. If the_post_thumbnail() is used you can use the post_thumbnail_html filter to adjust the attributes.

Also note that when you test on a HD/Retina device a larger (2x) image will be loaded if available. And some browsers won’t load the applicable smaller image if it already has a larger version in its cache. So make sure you test with an empty cache / cache disabled.

I posted in the W3TC support forum: https://www.ads-software.com/support/topic/cdn-doesnt-rewrite-srcset-urls?replies=1#post-7448963

As far as I can see only W3TC can fix this and there is nothing we can do.

Here is the link to the ticket for this issue in the RICG plugin repo on GitHub: https://github.com/ResponsiveImagesCG/wp-tevko-responsive-images/issues/88.

Probably unrelated but I wonder how the second source in the srcset can be a 1024×768 image with a w value of 600: https://squazz.dk/wp-content/uploads/2014/10/SAM_9652-1024x768.jpg 600w

The display width of image-size “large” (1024px by default) is limited to $content_width (600px in this case). However, the plugin uses the array with sizes that is returned by wp_get_attachment_metadata to build the srcset. This contains the actual width and height of image-size “large”. So it should be 1024w instead of 600w.

I haven’t been able to reproduce it with the current code though.

Joe, do you have any idea how this is possible?