There are surprisingly few differences between Mastiffs bar the glaringly obvious one, their size. For instance, the English Mastiff is the largest dog breed recognized by the American Kennel Club and can grow to up to two hundred and fifty pounds, which makes it a far larger dog than the Bullmastiff. Bullmastiffs also tend to be the most playful and energetic mastiff breed, which makes them a little more demanding than their English cousins.