A hand is four inches so they are usually under five feet tall at the shoulders.
Small size is more advantageous when trying to survive dry and cold and snowy conditions.
It takes less food to fuel a small, compact body than a big one. Palominos and buckskins as well as paints are found throughout the West. Wild horses have very hard feet. Obviously, they don't wear shoes so their hooves, which are simply that last toe of a once multi-toed foot, must be able to withstand all kinds of natural surfaces including the rugged mountains of the Rockies and the Sierra Nevada. Most are medium or heavy boned to withstand the rigors of running wild.