Bullmastiff's breed standard is Dogs, 25 to 27 inches at the withers, and 110 to 130 pounds weight. Bitches, 24 to 26 inches at the withers, and 100 to 120 pounds weight. I had a Bullmastiff before, and he now lives with my Uncle he weighs in at 153 lbs. at 3. I think 45 lbs sounds a little on the light side, but put him/her on some good feed and watch the pup flourish.