It’s just a marketing thing. Ideal nutrition being equal, large breeds don’t need anything different in their diet than small breeds do. You’ll notice that the best quality kibbles, Solid Gold, Innova, Canidae, etc, do not offer breed size formulas. That’s because they’re not necessary. Iams and such offer them because the more shelf space they can take up with their products, the better.
That having been said, when choosing a kibble, ivylass is right about the top ingredients being meat, rather than grain, but I’d take it a step farther and say you want to find a kibble where the top several ingredients are meats and/or meat meals, and do not buy a dog food that is made using ground whole yellow corn. Corn is, for the most part, completely indigestible to dogs and is implicated in a lot of allergy issues.
Dogs are carnivores, despite what you’ll hear from the kibble companies. At best, they can be called opportunistic carnivores, but they are still carnivores. Their digestion system is short and simple and highly acidic–in other words, designed entirely to digest meat and bone. Dogs do not produce amylase or cellulase, and cannot break down or utilize the nutrients in plant matter without mechanical help. Their jaw structure does not provide for a side-to-side grinding action, and their teeth are tearing, shearing teeth, not flat grinding molars. Thus, you want to provide a diet that has as little grains or plants as possible.
Which leads me to the raw diet. A species appropriate raw diet is the best possible nutrition you can provide your pooch, especially large breeds. Kibble diets often promote too-fast growth. A raw diet provides the right nutrients in the right form for a more natural growth rate–pups reach physical maturity roughly around the same time they reach mental maturity, which is how it should be. Adult dogs need the same nutrients, regardless of size. Everything your dog needs for perfect health can be found in a raw diet that approximates the prey model, that is to say, roughly 10-20% bone, 70-80% meat, and 10% other stuff, organ meats, eggs, table scraps, etc. The idea is to emulate what wild canids eat. It’s really, really easy, doesn’t take any more time than feeding kibble, and the cost is roughly the same. There are a ton of physical and mental benefits, mainly being clean and healthy teeth and gums, beautiful coat, chewing needs satisfied, about a fifth of the stool volume produced, reduced or eliminated allergies, more energy, strong bones and joints, etc.
Some people really freak out at the idea of feeding raw meat and bone to their dogs, and I always think it’s funny–no one ever tells people they ought to stay away from all that fresh, healthy stuff and eat more processed foods
And ivylass please be very, very careful with cooked bones. Raw bones are soft and pliable and easily crushed into tiny bits–cooked bones are just the opposite, they are hard and brittle and easily splintered, and can be extremely dangerous.