When do male Muscovy ducks get bigger than other ducks? We have 3 Muscovy ducks, 15.5 weeks old, and they are all still smaller than our other ducks, all full grown, Welsh Harlequins and a Black Swede. If they're males, should they be bigger by now? We did have to clip their wings a couple of weeks ago because they were flying quite well. So now they're stuck safe at home, but we still don't know if they're boys or girls!