Because he is the master of the vast universe, therefore he is called Maheshvara. The whole universe is pervaded by him in various forms. His mouth roars and burns the waters of the sea in the form of the huge mare's head.
महेश्वरश्च लोकानां महतामीश्वरच सः। बहुभिर्विविधै रूपैर्विश्वं व्याप्तमिदं जगत्। तस्य देवस्य यद् वक्त्रं समुद्रे वडवामुखम्॥