There is times when they hatch a day apart and one grows a bit faster, or you have a Hen and a Cock in the nest. As long as the crop is full and they are healthy, I do not think you have anything to worry about. Around 20 days they will be hard to distinquish. If the size is dramatic the larger may push the smaller out of the nest or keep it from eating. Check the crops. Now if something is wrong you will know it very quickly.