They will gradually get bigger. It is not usually overnight but a gradual process. Then after their first adult molt, you will see a decent jump in size. Why are they so small? They are still babies. Their bodies need to grow up and mature or they could hurt themselves laying great big eggs regularly. That's why I hate to see double yolkers with young hens. It does not happen often, but they are more at risk laying those big eggs when they are so immature. I'm just glad they can usually handle even the big eggs as well as they do.
I always enjoy watching those first eggs evolve and some times you get some really tiny ones. Once I got got a tiny one with no yolk!
Small eggs are much easier on the young pullet. Actually, it is kind of scary when they lay the huge, double yokers. Agree with Ridgerunner
