I had the same concern with my dog Rex and kept comparing him to my other dogs who were much heavier. I could see the ribs on Rex and when he would lay on his side it looked as if he was starving to death. I tried to feed him more, (slipping him treats of our leftovers when the other dogs weren't looking) but it made no difference.
I spoke to my vet, and anyone else who had experience with raising dogs, and was assured that this was normal for VERY active German Shepherds. Much more so than my other two guys who tended to be calmer and tire out after a bit of play.
I was afraid Rex was running off his calories and thought I should stop playing fetch with him so much. But every expert I talked to said that this was normal. Some Shepherds are just more active and have leaner builds. In fact, most police dogs have this same type of frame, with lower weight, due to their energy and active lifestyle.
Rex is now almost 3 years old now and has gained a
little weight over the past year as he slows down (
just a bit) LoL. So don't worry! Just enjoy this phase of his life.
