Hello.
Broody is a hormonal and bred “trait” its caused by lots of hormones at essentially random, though some breeds go more broody then others.
No, a broody hen will even sit on golf balls. They don’t care what they are on most of the time, this alone does not determine if they are fertile.
Size pictures would help, there are extremely small bantams and larger bantams.
Technically, a rooster can mate any hen. But a matter if it gets fertilized, thats another story. Picture him on her back, is he to small to psychically do anything? The cloaca must touch, if he is to small he will be mating her back and nothing will happen.
Do you know the breed of your bantam roo? A bantam serama is much tinier then say, a bantam cochin so it does help.