The size of your system and modules are in Direct Current (DC) power, which is not usable in the home. The string inverter or micro-inverter converts this to Alternating Current (AC) power to be used in the home. The size of the inverter represents the AC system size. There are several reasons to oversize the system for the inverter. A couple include accounting for DC to AC power losses, and ensuring you are getting the most out of your inverter during low sunlight periods. Please check out THIS LINK for more detailed information.