I thought that was odd too. I know a lot of mastiffs that grow up with children and are really good. That was a big bull mastiff. Usually the English are the biggest.
A bullmastiff, as far as I know, is a bulldog/mastiff combination. They are smaller in stature than an Old English Mastiff (or American Mastiff) and I do not believe the poster that said it was 4 feet tall on the back. The dog may be obese, weigh 200 pounds.
My mastiff is 36-37 inches tall at the shoulder. Weighs 200+.
I suspect someone is exaggerating here.