Why does the LESS mixin set background-size?

[maenu]

I wanted to use the LESS mixin to avoid writing media queries for high resolution images. When I just .retina('img.png'); and use background-size: cover; on the same element, but through another selector, which unfortunately has a lower priority, I need to resort to background-size: cover !important; to overwrite the background-size: auto auto; default of the mixin. Why is background-size set anyway, aren't @extras enough?

Edit I see that background-size is required as setting background resets the size. But how should I cope with my problem description?