When comparing two organisms of similar shape, which size organism (large or small) would be able to diffuse a greater amount of gas per unit volume through the surface of its body?

The small sized organism would be able to diffuse a greater amount of gas per unit volume through the surface of its body.

Explanation:

When two organisms have the same body shape, the smaller organism would have a higher surface area to volume ratio than the larger organism. It means that the smaller organism would have more surface area available for the diffusion of gases per unit of volume. Since the diffusion of gases or any other substances is increased with increased available surface area, the smaller organism would be able to diffuse more gas as compared to the larger organism.

As the body size increases, the surface area to volume ratio reduces which in turn reduces the rate of diffusion of the substances.