The intent was to get a smaller more aggressive Mastiff for a herdsman's night dog to drive off predators and subdue poachers.
In final composition it is (IIRC) 3/4 Mastiff and 1/4 Bull dog.
They are a fine dog. In a lifetime of dog ownership no breed has impressed me like a healthy active Bull Mastiff.
So the Bull mastiff is smaller and more aggressive. Our neighbors formerly had 2 old dogs that weighed close to 200 pounds each and they made me nervous just because they were so huge. They have 2 young ones now and I am careful to give them treats through the fence so they will like me. :D