Water expands as it freezes. Thats why roads up north get mondo cracks in the winter - liquid water fills cracks and the force of the expanding ice expands small cracks into big ones. Place a bottle of water, capped, in the freezer and check it out after a few hours. Use a plastic bottle.
Oops! I guess I was using junk science to arrive at my conclusion. Considering that the human body is made up of mostly water and considering how certain bodily appendages, (which shall remain nameless since this is a family friendly site) swiftly shrink in direct proportion to the drop in temperature, it just seemed logical that water shrank as it froze.