It seems that the early statues are small and made of more local stones (limestone, steatite and redstone); later, when wide-ranging trade-connections had been established, the more costly exotic diorite was used. Unlike the local stone, diorite is extremely hard, and so difficult to carve.
Early statues were small and crafted from local stones (limestone, steatite, and redstone). Later, as trade routes expanded, more expensive exotic diorite was employed. Unlike the local stone, diorite is exceptionally hard, making it difficult to carve.