The image size on WordPress is being set by the srcset attribute. The image that is published on Discourse is the image that is in the src attribute of the WordPress post. If you view the page source on WordPress (select ‘view source’, not ‘inspect element’ on your browser) you can see that this is the image that will be used by Discourse. It’s width will be set to 500px by the value of it’s width attribute, so it will be blurry.
For images where the size is important, you could edit the image tag on WordPress so that a larger image is set in the src attribute.