What determines adult size?
As mentioned above, the biggest factor in adult size are your dog’s genetics. Environmental factors only have a small impact on your Rottweiler’s final weight.
Genetics
Many owners assume that their puppy will be right in the middle of the mother’s and the father’s weight. This is not always correct! Size is not inherited in a straight-forward way. It can skip several generations and suddenly pop up again.
For example – your Rottweiler pup may have come from parents who both were of an average height and weight. The grandparent generation however had an especially tall dog. This trait can skip one generation and then be seen in the grandchildren. Your puppy might turn out to be an unusually tall and heavy Rottie, because he inherited that trait from 2 generations ago.
Intestinal issues
Certain intestinal diseases can stunt the growth in puppies. This especially applies to any kind of worms (hookworms, roundworms etc.) or a heavy case of giardia. The good news is that as soon as you treat the puppy, he will catch up on his growth. If you notice that your puppy has loose stools or does not gain weight even though he is eating plenty, get him checked out by a vet!