This is a good question. As you point out, according to the ideal gas law (PV=nRT where P=pressure, V=volume, n= amount such as moles, R= the ideal gas constant, and T= absolute temperature) when you cool a gas it contracts; yet, as you also point out, when you put a can of soda in the freezer it expands. This is because when the water in the soda freezes, it expands. Water is a very unique liquid in that unlike most other liquids, the solid phase is less dense than the liquid phase (this is why ice floats). So, when you put a can of soda in the freezer, the water in the soda freezes and expands which causes the can to expand. The reason water expands when you freeze it is because water molecules are tetrahedrally shaped and make up to four hydrogen bonds with the neighboring water molecules (two hydrogen bonds are donated and two are accepted). This forms a network of solid water which is less dense then the liquid phase where in some of the hydrogen bonds have been melted and the water molecules tumble around at a higher average density.