Just to add my sixpenneth, I have a chicken, ISA brown, that also says eggs the same size as the muscovy which lays larger eggs than the other muscovies!! Also, a muscovy that I had laid duck-blue-coloured eggs! Unfortunately a dog killed her, so I didn´t get any more, I´d have liked to have kept a muscovy that lays blue eggs, like the chickens...apparently the blue is caused by a virus in the predecessors that changes the colouring in the egg in the following generations. I imagine the same must have happened with the duck.