Horses are much larger today than their two-foot tall ancestor, mesohippus. Geologists believe the change in size was the result of changing environments, from tropical forests to a grassland habitat. Larger size would be an advantage for survival in this new habitat because

The answer is: larger horses have longer legs.

The ancestors of horses lived in the forest, where they could hide from the predators. But, with the first steppes, they could be spotted by the predators. Horses had to adapt in order to survive. Thus, their legs became longer through time so horses could run faster to escape predators.