it's a layout thing.

basically, there is a dimension ratio where the width has to be a certain size in order to support the height of a certain size.

change the ui scaling and it should change. you could also use an addon (like bongos) to move your action bars around.

alternatively, you could spin your monitor 90 degrees into portrait mode and do a horizontal split... giving you 2 very nice screens, one above the other. (if you do that... please post a pic )