Why is heat capacity an extensive property?

Heat energy is the internal energy of a substance. This includes kinetic and potential energy. It is impossible to accurately measure all the kinetic energy of the moving particles in a substance. The more particles you have, the more heat energy you have, thus heat energy is an extensive property. As a reminder, an extensive property of a substance is one where QUANTITY does matter.

capacity is an intensive property. No matter how much aluminum you have, its ability to absorb heat energy at a particular temperature is the same.