But none of them attained the size of their largest herbivorous contemporaries, or even approached it.

Among the dinosaurs on the other hand we find that--setting aside Brontosaurus and its allies as aquatic--the predaceous kinds equalled or exceeded the largest of the herbivorous sorts.

The difference is striking, and it does not seem likely that it is merely accidental.
The explanation lies probably in the fact that the large herbivorous mammals are much more intelligent and active, and would be able to use their weapons of defense so as to defy the attacks of relatively slow moving giant beasts of prey, as they do also the more active but less powerful assaults of smaller ones.

The elephant or the rhinoceros is in fact practically immune from the attacks of carnivora, and would still be so were the carnivora to increase in size.

The large modern carnivora prey upon herbivores of medium or smaller size, which they are active enough to surprise or run down.