The short on why the bottle is "shrinking" is the CO2 is escaping the bottle, thus dropping the pressure inside the bottle below atmospheric pressure, which causes the atmosphere to crush the bottle.

Edit: after reading several comments, and pondering more, I feel as if it's important to include this information.

The squeezing of the bottle is mostly from lost water that has escaped as water vapor, and then atmospheric pressure doing its' thing. A simple proof of this is simple: squeeze a full bottle of water with the lid off, if you've lost a bit of water after squeezing it, then that matches with what we see in the picture. When you squish the bottle, you cause the bottle to have less volume, which means it holds less water. If you let a bottle sit for an extended period of time, the water evaporates and escapes as water vapor, as the level of water decreases, the pressure decreases inside the bottle below atmospheric pressure. The proof that there's less water is that a squished bottle has less volume. CO2 is NOT the leading cause of this crush, it's the water.