La sortie de secours

Goku saves Vegeta from being absorbed. As Vegeta recovers, Goku continues to fight the thought form of Buu. Goku is rapidly losing energy, and just as Buu is about to destroy him, Vegeta cuts down the pod containing the original Majin Buu. This causes Buu to go mad, and so Goku and Vegeta grab the four pods and try to make their escape. On their way through is body, they notice one of the tunnels that leads to one of the holes in Buu's body where the steam escapes. They are able to fly out through here, and when they emerge on the other side, all six of them regain their normal size. As Goku and Vegeta set the pods down, they notice Buu undergoing a change. He shrinks into Kid Buu.

(Source : Kitsu.)