Sexual dimorphism is a strange thing.  Some creatures don’t have it at all, and even careful observation cannot tell the difference between the sexes.  Some creatures only have one or two key features that distinguish the sexes, but otherwise follow the same basic size and shape.  Other creatures will have such extreme differences that at first glance it is difficult to even tell they are the same species.  Humanity is an especially extreme case where the different sexes don’t even typically occupy the same habitat.