It just occurred to me that perhaps the solution to keeping bantams in a standard flock would be to have a partitioned run, but to treat the bantam side as a "panic room" with it own bantam size access. That way the bantam flock has full use of the entire run, but still has their safe refuge...
Is this bully hen very young? Barely past pullet stage even? If so, this is entirely normal behavior. The youngest flock members who were only just recently the target of bullying themselves for being new and at the bottom of the pecking order now find themselves in the position of being able to...