The process of diffusion is the movement of a substance, or molecule from an area of high concentration to an area of low concentration. Through this spontaneous process no cellular energy is needed or used and the rate at which it diffuses depends on the permeability of the layer that separates the higher from the lower concentrated areas.
Diffusion can be very effective over smaller distances. So, it is more productive in smaller organisms. The reason is as the ratio of surface to volume decreases, diffusion becomes less systematic. The best way to counteract this limitation larger organisms have developed specific systems and tissues that have taken the role of completing nutrient and waste duties.
Other limiting factors over organisms can be anything that limits its size, or stops its growth. These factors can include competition for food and other resources, and even the environment.
Endothermic organisms can maintain steady and specific body temperatures through the use of homeostasis since they rely on internal metabolic heat, for example mammals, such as humans, or birds. Ectothermic organisms rely on the environment to maintain body heat, and example of these are amphibians or reptiles.