Big banners are promotional. As a new addition to the visual landscape, they are by their very nature a demonstration of something new. The bigger the new banner the bigger the sense of promotion. Bigger is louder. Signs in the window are restricted to the size of the frame.
Banners have no such restriction.
They can be as big as the building. Big banners offer flexibility. You can change a banner. A new banner can have a new message. A new banner can have new colors and new dimensions.