Chicken size varies among chickens of a given breed, and it can produce noticeable size anomalies. As long as a runty chicken is behaving normally, no signs of physical distress, normal appetite, there isn't much to worry about.
Small chickens in their first year often catch up by age two, filling out and losing the noticeable size discrepancy. As egg layers, their eggs can be the same size or even larger than larger hens of the same breed.
If this is a failure-to-thrive chicken, you would be noticing physical issues such as crop disorders, lethargy, and a noticeable physical behavior such as droopy wings held away from the body and a fluffed appearance.