The frozen balloon shrank because the average kinetic energy of the air molecules in a balloon decreases when the temperature decreases. This makes the molecules move more slowly and have less frequent and weaker collisions with the inside wall of the balloon, which causes the balloon to shrink a little. This can be called contraction.
The information that the coefficients of a balanced equation give about the reactants are to tell how many moles of reactants are needed and how many moles of product can be produced.
