Scientists still have yet to figure out why small dogs live longer than large dogs. It's the same with every species, dwarf mice live longer than regular mice. They eat the same amount per gram according to body weight. They do get tumors, but they come late in life and develop very slowly. Scientists aren't sure why yet.

It wouldn't make any sense that it's because they don't herd and do agility. LOTS of small dogs do agility, and some small dogs herd. Corgis herd. And terriers dig, which is alot more physically demanding than herding and agility. And good physical strength makes you live longer, not shorter.