Domesticated animals which have been rapidly selectively bred to decrease and or increase size are a mixed bag of traits and do not really represent this trend very well.
Large dogs are breed to be large as well as have certain other traits without regard to longevity, so the kind of robustly persistent telomeres traits are not part of the selection process.
Though different breeds of dog are of the same species they are so far removed from the wolf that their genetic rate of entropy has been altered. Now when talking about two wolfs from the same genetic line, it may be true that the larger members of the pack will exhaust their telomeres much sooner and die earlier as a result(on average)
Just as larger bodied Humans from the same ethnic and environmental conditions will age faster than their smaller counterparts on average, though there are many variables.
Then when talking about cross species age differences the whole telomere frey hypothesis must be thrown out the window. for example.... All Mammals evolved from small shrew like creatures that only lived for around three years or so..... Blue whales the largest animal on the planet are descendants of these pinky sized animals yet somehow they are able to live 70-90 years( the oldest reported to be over 110) If genetic wear and tear was some kind of universal constant then there would be no way for an animal of such size to maintain itself for over a century.
There is indeed other forces at work which hold the ends of chromosomes together much more effectively in some of the higher mammals, especially those whom are a part of multi generational family groups and have a strong need for the guidance of the elders. Perhaps there is even a Will to survive phenomenon that the more consciously intelligent warm blooded beings evolved(humans, whales, apes, African Grey parrots,) that has allowed them to out live many of the lower metabolism creatures of smaller size.
