It's one of the more counterintuitive facts in biology: within a single species, smaller usually means longer-lived. A Chihuahua can live into its late teens, while a Great Dane is often considered elderly by age seven or eight. Across nearly every other species — elephants outlive mice, whales outlive minnows — bigger animals tend to live longer. Dogs flip that pattern almost entirely.
The leading explanation researchers point to is growth rate. A large-breed puppy has to pack an enormous amount of growth into a very short window — a Great Dane can go from a few pounds to well over 100 in about a year and a half. That kind of rapid cell division and tissue growth is metabolically demanding, and several studies have linked faster early growth to higher rates of cancer and organ strain later in life. Small breeds simply don't grow as fast or as much, so their bodies don't accumulate that same early wear.
Large dogs also have to maintain more total tissue over a lifetime — more heart muscle pumping blood over a larger circulatory system, more joint surface bearing weight, more cells that can potentially turn cancerous. Cancer is, in fact, the leading cause of death in large and giant breeds, and some researchers believe it's tied directly to simply having more cells that could go wrong.
This is exactly why a single "multiply by seven" rule for dog years never worked. A one-year-old Great Dane has already done most of its physical growing and is closer to its adult self than a one-year-old Chihuahua, which still has months of growing left. Then the pattern reverses in later life — the small dog stays in its stable "adult" plateau for years longer, while the giant breed enters its senior years by six or seven. Any calculator that doesn't account for both ends of that curve — fast early maturity and fast later aging in large breeds — will get the estimate wrong in both directions.
If you want to see this play out with your own dog's numbers, the pet age calculator on this site adjusts for exactly this — different aging curves for toy, medium, large, and giant breeds, rather than one flat formula for every dog.