Am I sinful for the increased number of chicken pieces I used to receive from KFC, whether the increase was from my colleagues' share or by another unknown source, given that I do not know the exact source of the increase?

What your friend offered you from his colleagues' share incurs no blame upon you. However, regarding what you suspect or are certain reached you unlawfully, you must question your friend about it. If he informs you that it is their right, then there is nothing upon you. But if it is not their right and he specifies the number for you, then its price must be returned to them. If he cannot specify, then let it be based on what is most likely, and caution is more preferable to clear oneself of responsibility.