The fossil record shows that early species of horses were generally in size, but many lineages since about 30 million years ago exhibit substantial change toward in size.

The fossil record shows that early species of horses were generally small in size, but many lineages since about 30 million years ago exhibit substantial change toward an increase in size.

Explanation:

It was demonstrated that variation in size as well as in diet during the last millions of years, were consequences of different climatic and geographic changes.

The fragmentation of horses´ populations was associated with habitat heterogeneity and to a new tendency for colonizing new areas that were not accessible before that time.

When great plains with grasslands appeared, horses moved to these new areas, reproduced, and got bigger and taller with time. The new species became even bigger. They evolved from small mammals that fed on shrubs in the forests, to the big current horses that graze on the open plains.