I mean that there is a media query set in the CSS for this theme that instructs the browser to use an alternate set of formatting selectors when the screen resolution of the User’s device goes over 768px. A typical approach for responsive design is to stack the contents of columns to avoid making the font size of the text content in those columns too small to read on a device. So, this probably means that your theme is performing as designed.
Many themes handle the scaling of a large header image by providing a JQuery-based slide carousel. If you just load the one slide in the carousel and shut off the slide navigation then it behaves like a header image that scales. I don’t know your theme, you might check and see if this is a possibility.