Just as Andy is about to pleasure himself with his brother's helpless body, he suddenly feels his body changing back to his original scrawny form. Underneath him his brother's body is rapidly swelling back to his teen aged jock muscular form.
Andy tries to pound Jesse's ass but his cock is now a three in nub instead of a nine inch monster and Jesse is now so much bigger than him that Andy's puny organ dangles loosely in his brother hole....until his brother, realizing what has happened, tightens his glutes and squeezes mercilessly.
Jesse felt the giant weight on him lessening at the same time he felt his size and strength returning to his own body. Even better was the feel of his teen aged hormones flowing freely inside him. Jesse yawned and stretched and squeezed his ass tightly around his brother's diminished organ. He could hardly feel the three inch appendage in his ass but when he heard his brother's scream of agony he knew he was no longer the helpless brother.