Because he is the foremost of all the celestials, and because his dominion and acquisitions are very extensive, and because he protects the vast universe, therefore is called Mahadeva.
देवानां सुमहान् यच्च यच्चास्य विषयो महान्। यच्च विश्वं महत् पाति महादेवस्ततः स्मृतः॥