It must be a bantam, as it's so much smaller than the rest of my brown/white leghorns (McMurrays website doesn't say one way or another) It's the size of my silver Spangled Hamburgs.
I agree she's smaller because she's immature, but if there's that much of a size difference I'm wondering if your other birds are indeed Leghorns? could they be Welsummers and Rocks instead?
So in these pics she doesn't look that much smaller, but I swear in person the silver seems much more petite. All the eggs we've gotten have been white so far, but our batch of chickens is only about 17 weeks so most haven't started to lay.