How does the curved edge of a first baseman's mitt help catch a ball?

I have heard that the design goal is to help a first baseman scoop up bad throws, but I am at a loss at how the design accomplishes this goal. Why is a curved edge better for this purpose than a traditionally shaped mitt? Or is there a different design goal? I know that the first baseman's glove is bigger than a standard infielder's glove, but why not use just one of the outfielder's huge mitts?