Generally, smaller breeds have longer lives. The reason why is still unknown, though Kraus suggests that there are several possibilities, including that larger dogs may succumb to age-related illnesses sooner. Also, larger breeds grow from puppies to adults at an accelerated rate, and this may lead to a higher likelihood of abnormal cell growth and death from cancer.
Of course, it is always just an average lifespan, it is not possible to predict, how long will your dog live. And there are a lot of things how you can actually prolong your dogs life, for example with quality food, not overfeeding them, giving your dogs proper exercise – mental and physical or just making their life happy.
There are some people saying, that mixed breeds have longer lives, because they are not inheriting the bad recessive genes. But there are no official papers on this subject, so it is still very unclear. And there are no official data about mixed dog breeds lifespan. But again, smaller mixed breeds generally have longer lifespan than large dogs.