Because of Kṛṣṇa’s weight, Tṛṇāvarta considered Him to be like a great mountain or a hunk of iron. But because Kṛṣṇa had caught the demon’s neck, the demon was unable to throw Him off. He therefore thought of the child as wonderful, since he could neither bear the child nor cast aside the burden.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
तमश्मानं मन्यमान आत्मनो गुरुमत्तया । गले गृहीत उत्स्रष्टुं नाशक्नोदद्भुतार्भकम्
tamaśmānaṃ manyamāna ātmano gurumattayā | gale gṛhīta utsraṣṭuṃ nāśaknodadbhutārbhakam
SB 10.7.27