The Arduboy is not only the same length and width as a credit card - it is also basically the same thickness too coming in at just 1.6mm thick! (and that's including the battery, speaker and OLED display!)
How did he do it? Well it's very clever really the maker of the Arduboy (Kevin) has designed his PCB so that all component bodies fit into their own special cut out in the PCB substrate. Since all components are 1.6mm or less in thickness, this makes for a very thin yet functional gaming device.