Isothermal expansion is an increase in the volume of a quantity of gas while the temperature stays the same. A constant temperature means that the volume of a gas is entirely dependent on pressure, which decreases as the size of the container increases.
The ideal gas law states that the volume of a gas goes up with increased temperature and goes down with increased pressure. The volume never changes unless one of the two other factors change. The pressure that determines the volume of the gas can be created by the walls of a container or a force that resists expansion such as gravity.