If the comet is much smaller than the full moon, then its surface will be brighter than the surface of the moon to still be of higher magnitude in brightness t

If the comet is much smaller than the full moon, then its surface will be brighter than the surface of the moon to still be of higher magnitude in brightness than moon. Magnitude is total visible luminosity of the body. Think of it as a logarithmic value of lumina. Total amount of visible light from that whole body that is visible to an observer on earth.