If I have learned anything about Mastiff's it is that you just never know. Oscar's mom is right that generally speaking the popular opinion is that they usually end up somewhere between the parents weight but while they are growing it is impossible to estimate what they may end up weighing. I have seen slow growing runts become very big dogs and fast growing big puppies end up smaller that the "norm". Rugers mom was 180 and his dad was 260 (Yes, he really was AND he was in awesome shape...Nice, trim, muscled, healthy boy). At one year, Ruger was 170.6 pounds and his last weigh in he was right at 240 pounds and 35 inches at the withers so he is not only a tall boy but a very heavy boned boy as well AND he is very "long". He will be 3 years old next month and is still gradually filling out but mostly done growing, I believe so I would say that he ended up his dads weight.