Domestic dogs vary in size much more that cats because they have been selectively breed for a much longer time. With a head start of thousands of years the relentless selective breeding of dogs has forced their sizes to new upper and lower limits. Today, the lightest dog might be 1.5% the weight of the largest, whereas for cats the figure is around 13% (miniature cat compared to F1 Savannah).
Domestic cats do vary in size quite considerably amongst the breeds. F1 Savannah cats are relatively large at more than twice the size of standard moggies and eight times the size of miniature cats. Random bred cats also vary in size, the smallest being perhaps around half to one-third the size of the largest.
The reason for this variation in cat breed size is selective breeding over many years. For example, miniature cat breeders select small cats each time and the size shrinks while F1 Savannah cats are wild cat hybrids, the father being a large wild cat, the serval.
The extended length of dog selective breeding is because the cat has been domesticated for a maximum of around 10,000 years while dogs have been domesticated for possibly three times that; 30,000 years (or longer). The dog was the first species to be domesticated.
It is believed that the selective breeding of dogs has taken place for as long as the cat has been domesticated. A study showed that 9,500 years ago dogs were selectively breed in northeastern Siberia as either hunting or sled dogs. This is the time when the first North African wildcats were informally domesticated.
There must have been informal dog breed standards even then. Purebred cat breed standards only exist in the cat fancy which started in the late 19th century.
Cat breeding officially started with the cat fancy in England. That being the case dog breeding has a 9,500 year start on cat breeding resulting in a much more diverse range of sizes and shapes among the domestic dog population.
The study proposed that the evolution of the domestic dog commenced some 30-40,000 years ago starting with natural selection, then the artificial selection of dogs that were the easiest to tame, and then selection of dogs for specific tasks supporting human activities.
Dogs were domesticated much earlier than cats because they were useful as working animals. The selective breeding of them tailored breeds to specific human activities. The domestic cat is seen primarily as a companion which today applies to dogs but originally the primary role was as a working animal.