Other source:
As the battle continues, Frieza decides to tell Vegeta that it was actually he who destroyed planet Vegeta, wiping out the Saiyan race. Vegeta claims that he knew all along, which was why he hated Frieza so much. Frieza decides to reveal another secret: he can transform. He claims that at full strength, his fighting power is over one million. Eager to finally destroy Vegeta, Frieza transforms into a hideous giant.
