Size and Proportion in Web Graphics.
Vertical space is scarce, too.
Fortunately, their efforts are greatly assisted by the incredible flexibility of our visual perception. Our eyes are highly efficient in determining the size of a rectangle with relevant information and successfully accommodating to it. After a very short while, we are no more disturbed by a web page being significantly smaller than the whole screen. This accommodation capacity could be compared to the lighting adaptability of human eye, which is a constant concern for photographers. Scenes that are mildly and quite discernibly lit to a human eye, are often too dark and have too much contrast for the film in a camera.