Big difference in size but they are the same age

She'll probably eventually catch up.

 

Some chickens are just naturally unthrifty; they may act normal and live happily but just don't get as big as the others. I once had a chick like that--she lived for 12 weeks, at the same size as a 4-week old, but died suddenly of an unknown cause. But as long as your chick acts fine otherwise, she should still be a great hen, and may eventually catch up to the others in size. Here is a link to an interesting thread on a similar bird: https://www.backyardchickens.com/t/...will-not-grow-update-hes-found-a-forever-home

 

Are you planning to use these chickens as meat birds? If not, the size really doesn't matter much.

 

I've had one particularly small one. She did grow to normal size and lays faithfully but is definitely slow mentally. But it's cute and she's a sweetie. So she may catch up in size.