Originally Posted by
pwnage101
One tends to compare two homes with their volumes, or two lenses with their focal lengths. There is a multitude of ways to compare any given thing, but we tend to use the convention. Like computer screens and television sets, photographs are measured using only one dimension. Naming the diagonal of a TV is no different than naming both dimensions of an image since they form a right triangle with all sides proportional. A rectangle with four times the area is said to be twice the size.
Remember, the aspect ratio is predetermined so the "other" dimension is implicit.
It was only with the introduction of digital photography that megapixels became a common marketing gimmick.