All mine are like that growing up, but they eat 4-5 cups a day (or more!). We're active, they're active, and they're healthy. It's just the way they grow. Mine often grow up (bony, skeletal) and then bulk out (and look normal!).
Don't be fooled by the dogs on TV, a healthy active dog should show rib when expanding their chest. They should have a defined waist and you should feel solid muscle when you pet them, not wobbly fat. Most folks keep their dogs a bit overweight because they think seeing ribs is bad; it's not! Don't worry!