Just tell her with a relaxing smile:

Just tell her with a relaxing smile: " Come on, poor girl, I know you DO love and want those leftovers, and you don't need to worry about me, I will be quite ok without them, however, I know you poor girl will be thinking about the leftover and may even bother yourself if someone else finishes it. So, go ahead, take it and eat it up, we will all be happy for you."