As the size of an organism increases, its surface area to volume ratio decreases. This means it has relatively less surface area available for substances to diffuse through, so the rate of diffusion may not be fast enough to meet its cells requirements.
Large multicellular organisms therefore cannot rely on diffusion alone to supply their cells with substances such as food and oxygen and to remove waste products. Large multicellular organisms require specialised transport systems.
The box on the left has a surface area of 6 square units and a volume of 1 cubic unit. Its surface area to volume ratio is 6:1.
The box on the right has a surface area of 24 square units and a volume of 8 square units. Its surface area to volume ratio is 24:8 which equals 3:1.
The larger box has twice the height, length and width of the smaller box, but only has half the relative surface. So the larger SA/V ratio of the smaller box would allow more efficient diffusion and exchange of materials.