https://phys.org/news/2018-07-pet-dog-years-cat-veterinarian.html

There's an old myth that one regular year is like seven years for dogs and cats. There's a bit of logic behind it. People observed that with optimal healthcare, an average-sized, medium dog would on average live one-seventh as long as its human owner – and so the seven "dog years" for every "human year" equation was born.

Not every dog is "average-sized" though so this seven-year rule was an oversimplification from the start. Dogs and cats age differently not just from people but also from each other, based partly on breed characteristics and size. Bigger animals tend to have shorter life spans than smaller ones do. While cats vary little in size, the size and life expectancy of dogs can vary greatly – think a Chihuahua versus a Great Dane.