Why is our 4 yr old male Lhasa such a bully to our 11 yr old female Lhasa?

She was here first and is 16 lbs. and he is a whopping big 26 lbs. He is such a menace to her a lot of the time! When they go out to potty and come back in he is so full of himself and aggravates the heck out of her. She will eventually snap back but he won't let up til I scold him. And then other times he lays with her like nothing has ever happened. We are afraid he is going to unintentionally hurt her seriously. He really is not mean at all...just like he wants to see what he can get by with. She is so humble and is like, " geez would you leave me alone already".