Considering all breeds evolved from a single subspecies, the gray wolf, dogs certainly are a varied lot. Differences in dog breeds range from body size and shape to facial features. Some are better at hunting and others are better at agility, while some cunning little canines seem to be masters at manipulation. With all these obvious differences between breeds, you might wonder about differences you can't see, such as brain size.
Some Size Variation
There is some variation in brain size among different dog breeds, but that is mostly due to variation in body size. W. Tecumseh Fitch's 2010 book "The Evolution of Language" explains that, in general, larger animals have larger brains -- you couldn't very well encase a humpback whale's 10-pound brain in your skull. But if you're comparing brain size relative to body size, as the breed's body size gets smaller, dog brain size doesn't necessarily shrink in exact proportion. Smaller dogs tend to have brains that seem huge in proportion to their bodies when compared to the brain/body ratio of larger dogs.
Brain Size and Intelligence
You might be tempted to jump to the conclusion that your Yorkie's relatively large brain makes him a genius. While his individual intelligence may be high, as a rule large brains don't necessarily equal high intellect. In the article "Bigger Not Necessarily Better, When it Comes to Brains" for Science Daily, Professor Lars Chittka says that many times a bigger brain can be like a computer with a bigger hard drive, but not better processors. W. Tecumseh Fitch reinforces "bigger not always better" by pointing out that there is no evidence that smaller breed dogs with their comparatively larger brains are smarter than larger dogs with relatively smaller brains.
Selective Breeding Effects
The selective breeding of dogs has undoubtedly affected the sizes of dogs' brains, and it's now thought to have even affected the structure of certain breeds' brains. The Scientific American article "Changing Minds: Has Selective Breeding Restructured Some Dog Brains?" reveals that dog breeds with shorter skulls and snouts such as pit bulls, shih tzus and pugs have their olfactory lobes located toward the back, near the base of the skull. This differs greatly from long-snouted dogs whose olfactory lobes are at the front of their brains. Even though the position of the olfactory lobes is different, lobe size is still the same.
If there's a difference in the size of brains among dog breeds, but bigger doesn't mean smarter, you might wonder which breeds are considered the smartest. The first thing to note is that individual dogs of any breed might show a higher intelligence than any other dog. Also, a dog who has a natural talent for agility isn't necessarily smarter than another breed of dog who can scent-track or hunt. But if you're looking for an official list of the top smartest dogs, it's satisfying to see that WebMD includes large, small and mid-sized dogs in their top 10: border collies, poodles, German shepherds, golden and Labrador retrievers, Doberman pinschers, Shetland sheepdogs, Papillons, rottweilers and Australian cattle dogs.