One of the reasons small dogs live longer on average seems to have to do with the time it takes for a dog to mature and the difference in size between their puppy and adult bodies. It seems that very large growth very quickly impacts a dog’s overall lifespan.
“Big dogs are known to grow and age faster than small dogs,” says Lefebvre. “For example, although it takes a Great Dane about 18 to 24 months to reach full size, that means they should be up to 32 inches (81 cm) and weigh 175 pounds (79 kg) during of this period. It’s remarkably fast, and physiologically demanding!”
Large breeds die of cancer more often than small breeds – a trend researchers believe has to do with how fast they grow in their first year of life. With all this rapid expansion, it’s possible that abnormal cell growth is more likely, or because their life is going faster, that they succumb to the diseases of old age sooner than, say, a Chihuahua. It takes about 10 to 12 months for a Chihuahua to grow to full size, but that size usually doesn’t exceed about 8 inches (15 centimeters) and 6 pounds (2.7 kilograms). It’s just not as taxing on the body to become an adult Chihuahua as it is to become a full Great Dane.
And another element related to the size? Obesity. Obese dogs are known to have a shorter lifespan than other dogs, much like human beings. “This is likely due to health issues like diabetes or cardiovascular disease from the lack of activity that accompanies obesity,” says Lefebvre.
Finally, lifestyle and access to veterinary care can affect longevity, and these factors may depend on the dog’s role in the family. For example, toy dogs may be more pampered than larger breeds, given their knee-friendly size.