It is not clear why the hunters were large. Even the biggest – Spinosaurus – could not have taken on a fully-grown plant-eater. However, they could have caught and eaten young plant-eaters, and large jaws would have helped to bite into the carcasses of dead adult plant-eaters.
One other reason for being large, or perhaps for being able to stand upright and show fearful teeth, was that hunters have hunting territories which they need to defend or they will not get enough food to eat. The more powerful you are, the easier it is to defend your territory.
