Because the reflection angle on your driver side is nearly 180 degrees, you get to use the full surface i.e. - the reflection you're looking at is, let's say, 5 inches by 7 inches.
Because the viewing angle from the driver to the passenger side mirror is far different much closer to 90 degrees, your actual viewing surface is nearly halved. Convex mirrors allow you a much better viewing angle because you don't have to turn them as much.