Since you are into behaviors, you don't have a rooster, you have a cockerel. Just like some teenage humans some adolescent cockerels can be a lot more mature that others, but 16 week is pretty young. It sounds like that Barred Rock is an older hen.
Some mature hens will squat for about anything in spurs but many mature hens require a male to act like an adult before they will accept him as a potential father to their children. Part of that is finding them food, dancing for them instead of just jumping them, keeping peace in the flock, being on watch for danger, things like that. Part of it is him having the self-confidence that he can wow them with his magnificence. Very few cockerels that young can even come close to doing that. I'm not sure yours is there yet but he may be getting closer than normal.
I read about size all the time on here. I find that size has very little to do with it. You may have heard the expression "it's not the size of the dog in the fight, it's the size of the fight in the dog". Same thing is true for chickens, it's not unusual for bantams to dominate full sized chickens. Size does give an advantage in a fight but usually the winner is the one that is most willing to fight.
It's not unusual for mature hens, especially the dominant mature hen, to beat up on a cockerel in a flock without a mature rooster. She's the boss, it's her flock, and no immature brat that can't measure up is going to come in and take over. Until that cockerel reaches a point of maturity he is just not that willing to fight her, even when he's bigger.
Several years I have a flock with mature hens and immature cockerels but with no mature rooster. I put my mature rooster in the freezer yesterday so I'm in that position again this year. My cockerel is 24 weeks old and not yet mature enough to take over. Sometimes this transition goes really smoothly, no real drama. The dominate hen will be in charge until he matures, then he will take over. Occasionally there is some drama when he takes over, some fighting between him and the mature hen, but usually not much. One year when he reached that maturity level, for him about 11 months, he spent two days going out of his way to attack the dominant hen before she finally accepted his dominance. After that they became best buddies. That much fighting is unusual. I think his slow maturity had something to do with it.
Most of my cockerels reach that point around 7 months, give or take, but a few mature earlier and some later. As you say watching behaviors is fascinating. Each flock is unique, each flock has its own dynamics. Just changing out one member can totally change those flock dynamics. We can tell you what we have observed with out flocks, we can even guess what you are likely to see, but its only a guess. With living animals you don't get guarantees when it comes o behaviors.