So, how big is a small dog's brain? The beagle, a small hound, has a brain that weighs about 2.54 ounces, or 72 grams, on average. So, how big is a medium-sized dog's brain? Or a large-sized dog's brain? While brain size differs from breed to breed, dogs usually have a brain-to-body mass ratio of 1:125. On average, for every 125 ounces, a breed ...