Using the Beagle as an example breed, the domestic dog has a brain with an average weight of 2.54 ounces, or 72 grams. Of course, this brain size differs by breed. Dogs usually have a brain-to-body mass ratio of 1:125.
On average, for every 125 ounces, a breed gains an ounce of brain mass. Smaller breeds naturally have smaller brains. However, it is important to note that brain size does not increase linearly with body size. Because of this, the 1:125 ratio does not hold true for breeds that are extremely large or extremely small. Smaller dogs have slightly larger brains than expected in proportion to their bodies, while larger breeds have larger brains that are slightly smaller than expected based on body weight.