Atmospheric pressure as a determining factor in animal height

The body size of animals depends on "external physical" factors and "internal physiological" factors. The length of the animals depends on the following factors:1-Air pressure2-Earth surface gravity3-Density of Blood4-Strength of the circulatory system5- Body angle relative to the ground6- Amount of organic phosphates (like DPG) existing in bloodOne of the important external factors is atmospheric pressure. Why does the amount of atmospheric pressure affect the body size of animals?