The paper presents a new approach to determining the change in the brightness of a comet during an outburst. It was investigated how the porosity of the dust particle translates into an increase in comet brightness during an outburst. It has been shown that the greater the porosity of a given particle, the greater the scattering efficiency, which directly translates into a greater amplitude of the change in the cometary brightness. In the case of dense aggregates with porosity ψ = 0.05, the brightness rate varies from −0.74 to −4.24 mag. In the case of porous agglomerates, the porosity of which is in the range from 0.30 to 0.90, the brightness change ranges from −0.91 to −6.66 mag.