Metabolism goes up with the surface area to volume ratio- meaning the more surface area and the smaller the volume, the faster the heart beat and the harder to keep warm. Big dogs have a huge volume and it is much easier to stay warm on a cold day for example-- my big Boo can swim in ice water and stay warm. A chihuahua I know died of hypothermia from swimming in August... It's just much harder to regulate the body temperature, so the heart rate goes up and more energy is used just in body maintenance. Finally these 2 years of Biology actually become slightly useful.
By the way, I emailed Solid Gold for the kcal/cup for the small dog food because it wasn't on the website and it's 380kcal/cup, compared to the big dog at 364kcal/cup.