Chickens can be mean! Mine chase squirrels back up our oak trees when then come down and I found a dead rat that got into our run and was peaked to death before.
I also agree with azygous ... some chickens are just smaller than others - even from the same hatch.
I would call the feed store and ask if their chicks are sexed or straight run. Straight run chicks are mixed male and female and sexed chicks are usually female (but its not a 100% guarantee). There is a thread somewhere on here where you can post pictures of your chicks around 5 weeks old and...
I recently raised a BR along with a RIR and an Orpington chick and noticed the BR feathered MUCH slower than the other two. This made her appear to be developing slower than the others but she is perfectly healthy and finally catching up in the feather department at week 5. I hope that is what...