Because in the wild, flocks are bigger so there are more girls for the guy (the dominant will kill or kick out the other roosters), there are no bantams to crush, and there aren't a handful of hens for him to rape, there are many many more hens, and they aren't half his size since they would...
Uhh... Just some friendly advise so your chickens don't die, if he is twice the size as a regular hen, that means your regular hens are about 4 - 5 pounds. They are getting hurt. So your bantams that are a quarter of the size is like a great dane mating with a purse dog. I would separate it...