Hi there,
thats correct – the way WP works is using src-set images, which creates various sizes of the Image, but they will all use the same aspect ratio. Various aspect ratios, known as Art Direction isn’t supported in the WP image function so having different ratios can only be achieved with CSS.
If you want to change the image sizes so they display correctly on mobile, then share a link to your site i can provide some CSS to change them for desktop.