A conductor isn't "prone" to a voltage drop -- all practical conductors have a voltage drop. The magnitude of the drop depends on the current flowing (I) and the resistance (R) of the conductor, and is equal to I*R. A (physically) larger wire size (e.g., AWG 12 vs AWG 14) has a smaller resistance per unit length, and so exhibits a smaller voltage drop. For example, over a 100' length of cable, an AWG14 cable carrying 10 amps will exhibit a voltage drop of 5.05V. Use an AWG12 cable instead, and the voltage drop is only 3.18V.
Excessive voltage drop can cause loss of efficiency in operation of light, motors and appliances. This could result in lights that are dim and motors or appliances whose life is shortened. So it is important to use the right gauge of wire when running wires for a long distance.
There's also a safety issue: The voltage that's "dropped" is actually used to push the current through the conductor, and in the process generates heat, the amount of heat roughly proportional to the square of the current. The more current, the more heat, and too much heat can be a fire hazard. Cables are sized and matched with circuit breakers so that the breaker will trip before allowing enough current to generate a dangerous amount of heat if everything is installed correctly.
So, if you try to run a big motor, say, on a 14AWG circuit protected by a 15A breaker, the breaker may trip at 15A. You might the up the CB to 20A or more just to allow your motor to run, but 2 bad things happen: 1) the voltage drop may result in a voltage too low to operate the motor properly, and 2) the excess current in the cable might result in a fire. For an overly dramatic demonstration of this, see .