small organisms have a large surface area to volume ratio therefore they don't require any special adaptations to their gas exchange surfaces as simple diffusion is sufficient
large organisms have a small surface area to volume ratio so there needs to be an adaptation to increase the efficiency of gas exchange
the lungs are adapted to maximise the rate of gaseous exchange by increasing the surface area available for diffusion, reducing the distance that molecules need to diffuse and maintaining a concentration gradient between alveoli and capillaries
