Behavior does not indicate gender. Some of the sweetest birds turn into roosters while aggressive ones can be dominate hens. Because your BO is larger and older, it may just be expressing dominance.
Here is something that might help you when your chicks reach 4 weeks of age, an age when they begin showing a difference in features.
According to UC Davis Veterinary Care Program.
2. Physical Characteristics (4-6 weeks of age)
a. Comb The cockerels comb is medium size and pinkish, the pullets is small and yellowish.
b. Legs The cockerels legs are sturdy and long, the pullets are finer and shorter.
c. Tail The cockerels tail is stumpy and curved, the pullets is longer and straight.
d. Back The cockerel has a thin line of stub feathers down the center of his back, the pullet has more advanced feathering along the center of her back.
e. Side of neck, flank and crop The feathering in the cockerel in these areas is poorly advanced, the pullets feathering in these areas is well advanced.
f. Wing bows In the cockerel the wing bows are bare, in pullets the wing bows are covered with small feathers.
I used this at five weeks and it looked like all were pullets even though one was very aggressive toward other chicks. They turned out to all be hens and "meanie" is getting less aggressive as she ages.
Good luck with your chicks. You will be an expert by the time they lay eggs!