Vaisampayana continued, “Being thus addressed by Bhima, the monkey with a smile showed him that form of his in which he had bounded over the main. And wishing to gratify his brother, Hanuman assumed a gigantic body which (both) in length and breadth increased exceedingly. And that monkey of immeasurable effulgence stood there, covering the plantain grove furnished with trees, and elevating himself to the height reached by the Vindhya.

In plain words: Hanuman smiles and, to please his brother Bhima, grows into the giant form he used to leap across the ocean. He becomes as tall as the Vindhya mountains and fills the whole banana grove.