Double coated German Shepherd
The difference is twofold. One aspect is in how many hairs are growing from a single follicle opening. For instance, the Labrador is a triple coated dog, which is why, though the fur is short, it is dense and oily, the better to stay bouyant in water.
A double coated shepherd will have the long slick guard hairs and a second, fuzzier undercoat. These hairs will look finer, but be curlier or wavier, making the slick guard hairs appear to have more volume.
A single coated dog will shed the fur in flat hairs, the double coated may actually look more like a molting, where the fur comes off in woolly sheets or clumps. When a dog "blows a coat", it is often the volumizing undercoat that is lost, giving the dog a slender, sometimes more bedraggled look.
To answer the second question, a big boned dog appears to have substance, thick set and nice solid legs, wide hips without being out of conformation with the breed. Dogs with fine bones are, for instance, the American Doberman, The Greyhound, The Border Collie. Dogs with big bones would be the German Doberman, the Great Dane or the Queensland Heeler. The entire frame appears to have more substance.