You've heard that one human year equals seven dogs years. The 1-to-7 ratio is not entirely accurate, however, as dogs of different breeds grow at varying rates.
Puppy to Adult in the Blink of an Eye
Even though larger breeds typically have shorter life spans than smaller breeds, the larger breeds reach physical maturity more slowly than their smaller counterparts. For instance, a Great Dane will be full-grown around 2 years of age and a Yorkshire terrier will reach full adulthood in approximately one year.
Varying Life Spans Across Breeds
In most mammals, the larger the animal, the longer the life span. In dogs, the fact is the opposite. Small breeds, despite reaching adulthood at faster rater, typically outlive larger breeds by quite a bit. This extensive variation in longevity is unique to the domestic dog; no other species has this trait.
Is Reproduction the Reason?
One primary theory as to why dogs reach maturity so fast is based on reproductive science. Animals who have shorter life spans, like dogs, reach sexual maturity much faster so they are capable of reproducing and continuing the species. Similarly, animals with larger litters and shorter gestation periods do not need to live as long to preserve the species, because they produce enough young to repopulate in a shorter time period.
Personality, Physical Traits and Working Proclivity
A recent study performed the by University of Sherbrooke tested the hypothesis that a dog's life span is influenced by the personality and working traits for which the dog was bred. A larger, stronger, potentially more aggressive working breed will have a faster metabolism than a passive or sedentary toy breed, thus it will expend much more energy in the same span of time, exhausting bodily resources and living a shorter life.