The amount of CO2 that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher compared to that of O2 because

According to Herny's law, the amount of a gas that will dissolve in a liquid is proportional to the partial pressure of the gas and its solubility. As the solubility of CO2 is 20-25 times higher than that of O2, the amount of CO2 that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher compared to that of O2.