Tuan Zhuan:

In Da Zhuang we see that which is great becoming strong. We have the (trigram) denoting strength directing that which denotes movement, and hence (the whole) is expressive of vigour. 'Da Zhuang indicates that it will be advantageous to be firm and correct:' - that which is great (should be) correct. Given correctness and greatness (in their highest degree), and the character and tendencies of heaven and earth can be seen.