It is generally accepted that small breed dogs have longer lives than larger breed dogs. Research has backed up this up by comparing the average size of different dogs breeds to their average lifespan and found that the bigger the dog, the shorter it lived.
The reasons why are not clearly understood, but it is an area which many scientists are exploring. It has been shown that in breeds with an average adult weight under 50kg the onset of ageing occurs at a similar time, but that larger dogs then decline more quickly. Dogs over 50kg start to age earlier, with an even more rapid decline. Large breed dogs have a higher risk of musculoskeletal and gastrointestinal disease, and are at higher risk of many cancers too. It is thought that selective breeding for dogs that will grow larger has altered their production of growth hormones, and that altered levels of growth hormones can increase the risk of these diseases. Several large and giant breeds also have a higher risk of heart disease which may be related to a difficulty in metabolising taurine and carnitine. Levels of inbreeding may also influence how long the average lifespan in a given breed is, and mixed breeds have been shown to have a slightly longer average lifespan than similarly sized pure-breed dogs.
There will always be exceptions to the rule! The oldest Great Dane is reported to have reached 15 years old, and the oldest Saint Bernard died just before his 13th birthday.
No matter what size of dog you have, you can maximise their lifespan by keeping them at a healthy weight, feeding a good diet, and taking them for regular check-ups with your vet. Ensuring they are well trained and socialised is vital too as the biggest causes of death for dogs under 2 years old are euthanasia due to behavioural problems and trauma (usually road accidents).