"She disguised herself as my niece, and fed off Cadence's fiance. Shining Armor loves her fiercely, as befits the Princess of Love. By the time the truth came out, she had grown incredibly strong."

Strong enough to overmatch Celestia, though Celestia has been restrained in other ways. Chrysalis had no qualms about unleashing her full force, while Celestia had her ponies and castle to consider.