I have 4 young Muscovy ducks. I think 1 male 2 female and one unsure. one big duck, and 3 smaller ducks, one is the same size as the 2 I think are female but its more aggressive then the big duck, it always raises its head feathers at me and at the bigger duck. I wasn't worried but today it was chasing and beating on the big one who was running and not attempting to defend itself. I don't know if he offended "her" over something, or was it male against male fighting starting? there was a 3rd duck watching this unfold and following them but not attacking anyone. So I don't know if I have a big male and a small male or just an overly aggressive hen. I bought these guys as ducklings and they are my first Muscovy so I'm a novice here. Also they were sold as pure Muscovy do they look pure or like mixes? I don't want mule ducks! They are 3-4 months old.
They look like purebred Muscovies to me. However, I'm not sure what sex they are. I agree with you that the larger one is probably a male, but other than that, I don't know. Hopefully someone with more Muscovy experience comes along and answers your questions!