I don't know what the mechanism for accelerating aging is in this case and it doesn't say, but IGF-1 serves a lot of purposes and it is ubiquitous among at least all mammals and I'm sure other animals I'm not familiar with. It is necessary for growth and healing and is what causes infants to grow and mature into adults. As with just about any molecule synthesized by a body, when its effect on mortality has been studied in humans, being either too low or too high are both associated with greater mortality risk. I'm guessing it's probably the same with dogs. Having higher growth hormone levels makes the larger breeds larger but also shortens lifespan. Nonetheless, being larger can clearly be important for a predator, especially in very cold regions where most available prey like caribou and bison and what not are very large. Shorter life might even be evolutionarily advantaeous on its own in places with food scarcity that can't sustain large predator populations. The old die to make way for the young as they'd otherwise starve them out.