This is because single-celled creatures have a very high surface-area-to-volume ratio, so have lots of available surface to exchange substances. Multi-celled creatures on the other hand tend to have a low surface-area-to-volume ratio , meaning their outer surfaces cannot exchange substances fast enough for all their cells.
The surface area of a sphere is given by the expression, . The volume of the sphere is given by the expression, . Dividing the surface area expression by the volume expression, we get, . So the ratio is inversely dependent on the value of r. Given for the sphere, the surface area = 588 m², and the volume = 1372 m³.