Why a Chihuahua is yay high and a Great Dane is bigger?

Scientists have identified a genetic marker that could explain why dogs have the widest range of body sizes among mammals. Because of selective breeding by humans since they diverged from the wolf over, dogs exhibit a wide range of body types and behaviours. The origin of the genetic diversity between different breeds of dog had been unknown.

Portuguse Water doge (may not always have its arse shaved)

An international team led by researchers from the National Human Genome Research Institute (NHGRI), part of the US National Institutes of Health (NIH),Researchers studied DNA sequences of more than 3,000 dogs from 143 breeds to pinpoint any gene variants that may help explain the size difference. In particular, they looked at Portuguese water dogs, which have the greatest variation in size of any individual breed.

They found that the smallest Portuguese water dogs had a particular piece of DNA sitting in their sequence, next to the gene that codes for the hormone insulin-like growth factor 1 (IGF1). The IGF1 gene plays an important role in body size for dogs. The "small dog" variant suppresses the activity of the gene, inhibiting growth.

The same sequence of DNA was found in other small breeds such as chihuahuas, toy fox terriers and pomeranians. It was not there in larger breeds such as Irish wolfhounds, St Bernards and great danes, or in wild members of the dog family including wolves and jackals. Researchers believe the mutation happened early in the domestication of wolves and the resulting smaller dogs were selectively bred by humans because they were easier to maintain in crowded villages and cities and easier to transport.