Why is my younger brother so mean to me?

Me and my brother have a VERY complicated relationship. We get into fist fights ALL the time!! He always starts it with calling me fat( which I'm not I weigh 80 pounds) and a horse( idk) and all these horrible things an I just want him to stop but when I tell him to he says he is doing his job and tell me to atop provoking him? Idk how to get through to him so we can just stop fighting all the time. So please help me to where we can just get along for once... Any advice??(: