I always thought that humans had something to do with how big or small a dog is through breeding methods.
Now, according to this article, the reason there are small dogs is because of a stretch of DNA.
“Small dogs the world over can all trace their ancestry back to the Middle East, where the first diminutive canines emerged more than 12,000 years ago.”
(Photo) A stretch of DNA allows Gibson, a Great Dane, to look a pony in the eye — while Zoie, a Chihuahua, can fit into a purse. Photo by Deanne Fitzmaurice / Science