Why Is One Electrical Prong Larger Than The Other?

Canada and US wiring - To maintain the polarity of the neutral to the fixture. On a 15 amp receptacle the neutral is on the left side. When plugging a lamp fixture into the receptacle it can only go in the slots one way. This keeps the neutral to the shell of the lamp holder socket and the hot side of the cord to the center pin in the bottom of the lamp holder socket. By using this configuration a person won't get a shock if the shell of the lamp holder accidentally gets touched.