Chick not developing as fast - is she okay?

[DenverStacy]

Hello! I'm new to chickens and we just got 4 chicks last Thursday, so they are 8-9 days old. I got 4 different types and my Plymouth Rock is much smaller than the others (Buff Orpington, NJ Red, Americana). She has pastey butt last week, but I cleaned her up and she's been fine back there since. But, she is much smaller, her feathers are just barely coming in, she doesn't scratch like the others, her fuzz is a mess, and she mostly stands up instead of laying down. The other girls are fine with her and don't pick on her, but she doesn't look well. I've attached some pics. Is she sick? Do Plymouth Rocks just develop slower?

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[DenverStacy]

 

[DenverStacy]

 

[aart]

She may be a few days younger.
Is she eating and drinking....making good solid poops now?
Is she moving around ok, fairly active?

Where did you get the chicks?
I wonder if she maybe a bantam?