I've heard about that too, but I think the one pictured has lots of orangey-brown. The "rule" applies to chicks that are all yellow/light chipmunk with well defined dark markings around and beyond the eye. But I wonder if it's really that simple.....
Darling chicks! That really does seem a big size difference. They look at about the same stage in feathering, so close in age. I had a Welsummer chick with an umbilical hernia, and she was very slow to grow at first. Had a lot of catching up to do. Maybe Ethel had a bumpy start after hatching...