Strange Guppy Behaviour

[rahina]

I have a tank with a male and three females guppies (a big species in size sorry for I don't know the name) and a male and four female guppies (another species small in size) one of the big size females is bullying the other females her species and the other species (poor other females the big ones run but she still chases them and the smaller ones hide away what on earth could be the problem? the water is okay I do regular water change and have live plants