There is no definitive answer to this question as it depends on a number of factors, including the individual dog's size, weight and proportions. However, in general, Rottweilers tend to be slightly larger than Bullmastiffs. This is partly due to the fact that Rottweilers were originally bred as working dogs, while Bullmastiffs were bred as guard dogs. As such, Rottweilers typically have more muscular builds and are heavier than their Bullmastiff counterparts.