Electricity is a flow of electrons and conductivity is better in materials that give up their electrons more efficiently. A smaller conductor has less electrons to give up, current can't flow as well (the force - voltage) is assumed to be the same in this example. So, since Voltage equals Amperage times Resistance, if the voltage stays at 25 volts for big or small wire, and the resistance is 5 ohms (25 volts div. By 5 ohms) in the big wire and 10 ohms (25 volts div. By 10) in the small wire, the current is 5 amps in the big wire, and 2.5 amps in the small wire. So, the R doubled and the current was cut in half.