The reason why we do not show full width of the link image is because some of the image is not big enough, so we decided to make all the link image display consistently. If we make it full width, mean if those link image is not big enough, it will display like stretch the image, and the image become ugly.
By the way, if you would like to achieve this, try apply this following css into your template css file and see how it goes.
#es .es-stream-embed.is-link .es-stream-embed__cover img {
max-height: 100% !important;
max-width: 100% !important;
width: 100% !important;
}