"One intriguing clue, gleaned from previous studies, is that small dogs have lower concentrations of the growth hormone IGF-1, or insulin-like growth factor 1, in their blood than large dogs.
High levels of IGF-1 has been shown in a wide variety of organisms, including humans, to be associated with increased risk of death from age-related diseases such as cancer and heart disease. Conversely, manipulations that lower IGF-1 levels in many animal species slow their rates of aging, reduce the risks of age-related disease and extend their lifespans."