The abbreviation "CC" after a number on a motorcycle means cubic centimeters. It is determined by the volume of the air and fuel mixture that moves through each cycle of the pistons.
A larger amount of fuel and air mixture in the cylinder causes a larger combustion. This increases the power of the engine and speed of the motorcycle. A larger cylinder is required to allow for more fuel and air mixture to be sucked into it. Larger cylinders mean a larger engine. This is why many people think that the CC designation refers to the size of the engine and not the displacement volume of the fuel and air mixture.