My boy Cody has a couple of grape-sized lumps on the outside of his ribcage. He has about 4 of them, but they are just fatty tissue growths, which can be quite common on older dogs. (Cody is 8 years). Also my previous dog Lucky kept getting benign growths on the back of his hind legs, we got them tested and removed after they continued to grow. Luckily it never impeded him in any way, and they posed no threat to his health whatsoever.
The only way to truly find out what they are is to visit your veterinarian and get them to run some tests. Chances are they are something minor that won't bother your dog, let us know how it goes!