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Well in either case it COULD happen...it's only physically difficult to carry out and critters (in both cases) might get hurt in the process. I mean, it would be physically difficult for a great dane to breed with a toy dog of any species simply because body parts don't line up. Same with your chicken example, plus there could be physical damage due to the weight/size difference. A 12 pound rooster would physically damage a 2 lb banty jumping on top of her (since he has no "thing", no issues there). A banty rooster trying to mate a jersey giant hen...no damage, but body parts just don't line up right, so most likely the eggs would not be fertile... But otherwise, yes.