I had a chick that also grew so much slower then her siblings, (She is the one in my profile pic) while she wasnt as small as your little one it still seemed like she was a week or more behind in size and feather growth compared to her sisters (and brother)
Here is a comparison between Squirt (the little one) and her sisters
Here was squirt at around a few days old
and Here is her sister Dottie at a few days old
and Annabelle
and Here are the same ones at a week later then the first picture.
Squirt, Dottie then Annabelle.
and Here they are 2 more weeks later
Squirt
Dottie
Annabelle
and now 9 months later squirt has really come into her own and is one of the 2 biggest hens (Dottie is the other) I have and she not only has the most unique feather coloring out of all them but also has some neat feathers behind her head.
Squirt now at 9 months old.
everyone together left to right Squirt, Dottie, Annabelle, Sparky (cockerel), Maverik, and Ziggy.