To fifty leagues he waxed, and still Her mouth grew wider at her will. Then smaller than a thumb became, Shrunk by his power, the Vánar’s frame. He leaped within, and turning round Sprang through the portal at a bound.

In plain words: Hanuman grows to fifty leagues, but her mouth keeps growing too. Then he suddenly shrinks smaller than a thumb, darts into her mouth, and jumps right back out.