Yes, I think you are right in that they grow very quickly and tend to look thin throughout their growing years. My 2 year old straight Anatolian pups still look a bit thin. They do fill out with age though. Give him time. Interestingly, our idea of a healthy looking dog is actually considered overweight to obese. There are some good charts online that show how to tell if your dog is too thin or over weight. Try googling "dog weight chart". I saw a chart in my vet's office and was surprised that a healthy dog actually looks quite thin in the tummy area. If you are concerned that your pup is too thin or he has any signs of illness, please check with your vet.