Genetics can be a result or just puppies need more food when they are in a growth spurt and the quality of food can make a difference in his size. Giving a pup a bowl of high quality kibble will help insure he is getting all the nutrients he needs and is growing as he should. Don’t be alarmed if your pup thins out a bit during this period despite extra food. Check on his growth stats and, once he slows down, he should fill out a bit again.