Rottweilers have a tendency to look scrawny and underfed in their adolescent period, as the breed tends to reach adult height faster than adult weight. Usually, Rottweilers don't reach their maximum weight until they are 3 years old. This may cause owners to think their dog is scrawny.
Avoid overfeeding a young Rottweiler to try and speed up the growth process. Overfeeding at a young age can lead to health problems later in life. Additionally, some healthy Rottweilers may not be as large as others depending on the dog's genes. The best estimate for the adult weight of a young Rottweiler is to take the average weight of the parents. It is normal for this adult weight to fall between 75 and 130 pounds, which is a large range. The notion that Rottweilers are supposed to be extremely large dogs is unfounded; the German Rottweiler Breed Standard defines them as being medium to large dogs.
For older dogs, however, being underweight may be caused by an illness, a reaction to a recent vaccination, traveling, unfamiliar surroundings or a behavioral issue. Because of the possibility of illness, it is important to consult a veterinarian if an older Rottweiler is notably underweight or has lost weight rapidly.