The Supreme Lord is the Supersoul of all the demigods entrusted with the task of constructing the cosmic manifestation. Being thus prayed to [by the demigods], He thought to Himself and thus manifested the gigantic form for their understanding.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
स्मरन्विश्वसृजामीशो विज्ञापितमधोक्षजः । विराजमतपत्स्वेन तेजसैषां विवृत्तये
smaranviśvasṛjāmīśo vijñāpitamadhokṣajaḥ | virājamatapatsvena tejasaiṣāṃ vivṛttaye
SB 3.6.10