While not unusual, it is not "normal" either, the human body tends towards being symmetrical but does not always happen. If you seem to be having health problems that could be kidney related, I would see a doctor. But if you have just noticed one is slightly smaller than the other with no other problems, it is completely normal. Also I would note that the other kidney could have a cyst, or a tumor and therefore feel larger, so getting a doctor's clearance might be a good idea anyway. Hope everything is good!
