You see, we must cast our minds back to Charles' law. Charles' law gives the relationship between the volume of a gas and temperature of the gas.
Now, Micheal left the balloon outside at a particular temperature and volume the previous night. Overnight, the temperature dropped significantly and so must the volume of the gas in the balloon!
Remember that Charles' law states that, the volume of a given mass of gas is directly proportional to its absolute temperature at constant pressure. Since the pressure was held constant, the drop in the volume of gas in the balloon can be accounted for by the drop in temperature overnight.
