Smaller animals have a higher metabolic rate which increases the amount of free-radicals in their bodies, shortening their lifespan

Smaller animals are more likely to be predated compared to larger animals, therefore smaller animals must invest more resources into early life development at the expense of longer lifespans

Smaller animals are more likely to struggle for food (given that they can't out-compete other organisms for food based on their size). This imposes a high level on stress on smaller animals

Larger animals are more likely to eat the smaller animals, therefore there will be less survival stress for larger animals and they must evolve a mechanism to live longer