Conceptually, I understand that in order for a chick to break out of its egg and become a seven-pound hen in four months, there's got to be an amazing rate of growth day in and day out.
Practically, I had no idea I'd literally be able to see the difference day over day.
I attributed the noticeable growth from Day 1 at our house to Day 2 as a simple matter of "they're just walking around the brooder bigger because they're more comfortable now that they've spent a night in their new home."
I can't attribute comfort-with-surroundings for the now-noticeable ridge of bumps that will eventually become the comb (cf this chicken anatomy illustration). They weren't there on Day 1.
I'm also starting to notice a difference in size between the two chicks, and I'm hoping that's just a genetic difference between the two pullets and not the early signs that I've got a "brother and sister" instead of two sisters. Stand by!