Smaller is better
On an open plain such as an African grassland a single herbivore is an easy target for a predator unless you are very large such as an Elephant (Loxodonta africanus), Black and White Rhinoceroses (Diceros rhinoceros and Ceratotherium simum) and Buffalo (Syncerus caffer). The dense vegetation of a forest makes it easier for a herbivore to move around without being seen, and a smaller size would tend to be favoured. On the plains a small size is not an advantage, in fact a large size may reduce the risk of predation. Great bulk with a tough skin may be deterrents to predation. However, for smaller animals, the dangers of predation are high.