“The size of the fetus at birth is often determined more by the mare’s uterine capacity than by genetics…”
http://www.thehorse.com/articles/10257/fetal-development-and-foal-growth – In other words during pregnancy the equine fetus will only grow to the size the mare’s uterus can accommodate. An example would be a 13 hand mare bred to a draft stallion would deliver a fetus appropriate to the size of her uterus. . Once the uterus has reached capacity, birth occurs, regardless if it has reached or has gone beyond the normal 11 month gestation period. This will not determine the size of the horse after birth. This ‘uterine size adaptation’ is apparently unique to horses & not found in cattle or other mammals.