Rats have a pack structure much like dogs. They do a bit of fighting to sort this out, and to reinforce it. Make sure the rats have plenty of room, and both have access to necessities. Watch the smaller rat for bad cuts and for weight loss. If these get too bad, separate the rats.
Otherwise, don't interfere. If you break up the fight and try to comfort the little one, it may just make the fighting worse. It should die down soon, although rats will always scuffle just a bit from time to time. But that's ok, they're usually MUCH happier together than alone.