Why is one of the calves bigger than the other of the leg?

proobably because you have a more dominant foot when yoou walk. You can test this by picking a point directly in front of you about 50 metres, close your eyes and try to walk in a straight line. If you drift off to one side, that side is the more dominant leg. To fix it, just becoome aware of it. If you want the other one to be just as big do some single leg rises. Just stand on one leg and rise up and down repeatedly. If you dont want it big, try to stop using it as much and once it gets sore, stretch it out.