Thereupon Hanuman addressed Bhima with a smile, saying, ‘O sinless one, thou art capable of beholding my size up to this extent. I can, however, go on swelling my size as long as I wish. And, O Bhima, amidst foes, my size increaseth exceedingly by its own energy.’

In plain words: Hanuman smiles and tells Bhima that this is as much as Bhima can bear to see. He can grow even bigger whenever he wants, and when enemies are near, he grows larger by his own power.