The Pai-Pai Rocket was one of Lala's bizarre inventions. After hearing one of her (perverted) gym mates say about men liking large breasts, she invented it so it can give girls their ideal breast size. The rocket increases the person's breast size by adjusting their hormone balance. However, not only does the rocket transforms a girl's breast size, it can even completely change a boy into a girl. Rito, for example, was hit by the rocket and was turned into a beautiful girl (under the alias Riko).
Later while bathing, Riko's boobs grew to an enormous size when she was embarrassed. This could mean that a girl's (via hit by the rocket) breasts change size depending on how they feel.