The age difference between the 'grown ups' and the Youngins is only about a month, but what a difference a few weeks make! Even when comparing one chick to another one sees size and development differences. One of our little princesses is about half the size of her sisters of the same age. She's not a banty, just more petite.
As you note from the picture, although they are all Americaunas their color characteristics can be quite varied. Their colors also make it difficult to spot them when they're wandering about in the high grass. At the end of the day, when everyone is back in the hen house, I always do a head count just to make sure that no one is left out for the night.