Our house's outlets and switches are all 12 gauge. However, while changing the color of our outlets, we saw that 4 of our outlets had black wires that were too big to backstab into the new outlet (I know now that we need to wind it around the screw, and not backstab), unlike all of the other outlets in the house. Does this mean these 4 black wires are 12 gauge? If so, is there a common reason for this? They're in the dining room. Can we still wind these wires around the gold screws on a 15amp outlet? Or do we need to put wind them on a 20 amp outlet?