The physical size of the chicken isn't important in things like this. The personality and spirit of the individual is much more important. It is pretty common for bantams to dominate full sized chickens. You cannot count on size, as you have seen.
ISA Browns are commercial laying hybrids bred to take confinement well and live in close contact with lots of other chickens. She is technically not a breed but close enough. Breeds may have tendencies but each chicken is an individual. Most chickens of a breed may follow certain behaviors but you can always find an exception. I don't find breed to be all that important in describing the behavior of an individual. You can find sweet chickens or brutes in any breed.
It is not unusual for chickens to pick on a weak or sick member of the flock. In the wild a sick or injured chicken can attract predators so they try to run them off or kill them to protect the flock. Our chickens are no longer wild so many don't do that but some still do. That may explain why the ISA Brown picked on that specific chicken. Or maybe she was starting her campaign to become the dominant chicken in the flock and decided to pick on an easy one first.
Do you know how old the ISA Brown is? How old are your chickens? A difference in maturity level can have a huge effect in this type of behavior and how to manage it.
How big in feet or meters are your facilities, coop and run? Photos could be beneficial in determining what specific recommendations I might make in your case. The tighter your space the more problems you typically have with integration.
I'm not sure what is going on or what you have to work with. All I can do is offer some generic integration methods that often help. House them across wire for a while. You did but not sure for how long. Longer might help but many times it doesn't, each situation is unique. Give them as much room as you can, both in the coop and outside. Try to not lock them into a tight space together but give the room to run away. You can sometimes improve the quality of what room you do have by adding clutter. Clutter means giving them things to hide under, behind, or over to break the line of sight. Have widely separated feed and water stations so they can all eat without conflict, especially out of line of sight of the other.
It is possible she is a brute and bully, not suitable for being in a flock. It is possible she is trying to become the dominant hen in the flock and will mellow out when she achieves that status. It is possible your facilities are so small it leads to brutal integrations. I don't know what is going on.
