Change in temperature causes change in volume
A change in temperature of a fixed mass of gas at constant pressure causes a change in volume of the gas.
If you put a balloon filled with air into a freezer, you will notice that it shrinks dramatically as it gets colder.
Increasing the temperature of a gas means that the gas particles move faster.
To keep the pressure constant the gas particles must hit the walls of the container with the same frequency.
This can only happen if the walls of the container are further apart - the volume is increased.