My Pit mix Gus was 12 pounds when I got him at about 10 weeks old, and very skinny, even though he was being fed pretty much all he wanted to eat. At 4 months, he was an even skinnier looking 25 pounds, even though he was eating a ton of food, and my new van's interior, the hall carpet, shoes, furniture, etc. At a year, he was a still very skinny 48 pounds, with huge feet, and short, thick legs, with a tiny "waist". At 18 months, he was about 60, and had suddenly gotten a couple inches taller, and was much less "Dachshund with a giant head" looking than he was when he was little. It took until he was almost 2 and a half before he hit the 70 pound mark, topping out at a hard as a rock 75 at about 3. He ended up having basically a Pit body, with a brindle and white Pit coat under a Golden Ret coat, and a big, hound type head with amazingly big teeth. Everytime the vet would see him while he was growing up, he would look different, with more muscle. Over the years, no vet, or tech ever got his weight right by guessing at it. They always said, 45, 50, maybe 55, and were shocked when his 75 would come up on the display. He was very compact, and felt like picking up a bag of concrete that moved. Some pups just don't follow formulas..