What does a lie keeps growing and growing mean?

It means that lies keep getting bigger. For example, if you and a friend were to go to a movie instead of sitting home and studying like you were supposed to, it would be a lie to tell your parents you studied. Then they might ask what you studied and you would have to lie about that, if it was interesting and you would have to give them an answer to that. Therefore lies keep growing and growing until it's as plain as the nose on your face.