Why do dogs’ skulls bulge?

Protruding skulls in dogs are normal and can be seen in large dogs such as golden retrievers, Tibetan mastiffs, and Labradors. This is also a sign of pure blood. The bulge is usually on the back of the head, and in some dogs the bulge is more obvious. For example, the purely British Golden Retriever has a raised skull, while the purely American Golden Retriever has a slightly concave skull.