Diffusion rates are dependent on molecular sizes because larger molecules diffuse slower than smaller molecules. The sizes of the particles involved in the diffusion are important because they closely relate to the concepts of heat and energy in the context of diffusion. It takes more energy and heat to move a larger object than a smaller one, so larger particles require more heat from their surroundings.
Diffusion relies heavily on energy and movement. In any given environment, there are several factors that impact a molecule's mobility: size of the molecule, viscosity of the surroundings, interactions between macromolecules and the specific type of transportation that is used by the molecule. Because diffusion relies on movement and movement relies on the size of the molecule, there is a direct relationship between molecular size and diffusion rate.
Larger molecules necessitate larger amounts of energy to engage in the same level of activity as smaller molecules. If there is not sufficient energy to move a larger molecule, it resists the effects of diffusion and is unable to move from one area to the next. Increases in environmental heat or other extraneous factors also have an impact on the energy involved in the diffusion process and the rate at which a large molecule diffuses.