SB 10.18.26

tam udvahan dharaṇi-dharendra-gauravaṁ
 mahāsuro vigata-rayo nijaṁ vapuḥ
sa āsthitaḥ puraṭa-paricchado babhau
 taḍid-dyumān uḍupati-vāḍ ivāmbudaḥ
Synonyms: 
tam — Him, Lord Baladeva; udvahan — carrying high; dharaṇi-dhara-indra — like the king of the mountains, Sumeru; gauravam — whose weight; mahā-asuraḥ — the great demon; vigata-rayaḥ — losing his momentum; nijam — his original; vapuḥ — body; saḥ — he; āsthitaḥ — becoming situated in; puraṭa — golden; paricchadaḥ — having ornaments; babhau — he shone; taḍit — like lightning; dyu-mān — flashing; uḍu-pati — the moon; vāṭ — carrying; iva — just as; ambu-daḥ — a cloud.
Translation: 
As the great demon carried Balarāma, the Lord became as heavy as massive Mount Sumeru, and Pralamba had to slow down. He then resumed his actual form — an effulgent body that was covered with golden ornaments and that resembled a cloud flashing with lightning and carrying the moon.
Purport: 

Here the demon Pralamba is compared to a cloud, his golden ornaments to lightning within that cloud, and Lord Balarāma to the moon shining through it. Great demons can assume various forms by exerting their mystic power, but when the Lord’s spiritual potency curtails their power, they can no longer maintain an artificial form and must again manifest their actual, demoniac body. Lord Balarāma suddenly became as heavy as a great mountain, and although the demon tried to carry Him high on his shoulders, he could not go on.