Han Sen's captured creature did not know it was being tricked. It thought it was being treated kindly, and so it happily gobbled up all the walnuts it was given. Before long, however, it started to writhe around on the ground in pain.
Han Sen was then able to again confirm that the walnuts made creatures grow smaller. Three days later, it returned to its ordinary size.
Han Sen then decided to eat the walnuts himself, anxiously wanting to enter the tree hole and see what may have resided inside.
The creatures always returned from the tree with the same or even greater strength than they had before. If Han Sen was to do as the creatures did, he thought he'd share the same boon or at least go back to his original size in three days with all his power still intact.