its worse than that, they will actually eat the fish, its just they sense a weakness they will all pick on the fish, the fish gets more and more overcomed and stressed, eventually due to stress, diesease will set in the fish get a free meal, esp where fish of diffent sizes are kept, also there is less competition for the food you feed,if there is one less mouth. Its just the law of nature and not much we can do other than to minimise it as much as possible, you always get one tank bully, just like in the playground, the others may try and keep out of its way.
The bigger the tank the less chance of it, the bully cant be everywhere at once so spread the food out as much as poss, also hiding places and well planted tanks will hinder bullys. I had a very bad one, that actually started to bite of bits of the other fishes.