My GSD put on about a pound a week. He was very likely ~12lbs at 12 weeks. I wouldn't put a lot of stock into whether or not your dog is GSD based on weight alone. Each pup is different and living in different circumstances. To be honest, the slower your dog puts on weight the better it is on his joints. I kinda worry about the super big, super young pups and their likelihood to develop HD from carrying around so much weight on such an immature bone structure.