Funny my largest hen lays the smallest eggs! Ha
I have raised quite a few blue cochins and always keep a few on the lot for no other reason than they are so dang pretty. Cochins in general are not the best layers though..usually medium to small eggs at a rate of about 3 a week. Maybe 4 a week if she is feeling friskey.
If your Orpington has feathered shanks..she is a cochin.