Grav images folder serves same file size and resolutions

I'm using a theme based on Grav 'quark' theme. This theme uses a background css image for it's hero image. I would love to serve different images relying on the responsive viewports. Anyone who could give me a lead?

Also when serving images to GRAV with @x4 extension ... it gives me always the same file-size and image resolution.

Kind regards,
Xavier

If I look in the inspector. The background image that is served comes from an asset that is created by Grav as well. It is in the 'images' folder but there is no different in file size or resolution.