Picking on the females only!

I have 6 indoor only cats........5 are purebred Ragdolls. All cats are either spayed or nurtured. I have 3 males and 3 females. I have one male that keeps picking non vicious fights with the females Brutus is about 15 pounds and is 2 1/2 years old. He prefers to pick on his own birth mother and another female former breeder cat mother. Both mothers are about 11 pounds in weight. Brutus will pick on the 3rd female that is sick from a kidney disorder and is a offspring of one of the mothers and only weighs 7.5 pounds. Brutus never hurts the girls....but he will chase them for 5+ minutes straight through the house before stopping Why does this happen????? I need some advice.

I have 2 smaller females that are picked on by the 2 youngest boys. Like you there is no real vicious intent on the boys parts, I think they just like to see the girls run away from them. I haven't found yet how to deal with this except to tell the boys a firm NO when they do it. I just let the girls find their own hiding spaces when the boys are in a bratty mood and I have some higher shelves that they can go up on that for some reason the boys won't follow them up there. I know its very frustrating and I'm hoping in time they will grow out of it. Good luck.