why does shelley use vast to describe the remains of the statue rather than big

"Ozymandias" - ???  The statue, although now decomposed, represented the rule of one of the great Pharoahs.  His rule covered a vast area of land and was thus not just large because of its size, but vast, because of its influence.

Please could you describe with more detail, this would help me a lot for my English homework. Thankyou