Size will always win over attitude in the chicken world. A RIR roo at 9 lbs., is three times larger than a bantam. Imagine an over "hormoned" human three times your size. Pretty scary! It may not happen this week, but it will happen as soon as the RIR figures out that he can flog the bantam and get his way.
I have a bantam D'Uccle and two EE roos... and I just built a new "condo" for Slash when he was out sized pretty much overnight. The problem was that the big boys didn't just prove their dominance, they kept going after Slash and pretty much beat the crud out of him.
Can he mate? Sometimes. If the hen is willing, and he gets his balance just right... anything is possible. In my pen, the percentage of fertilized eggs is far lower for the bantam.