While the designs of the bottles can be attractive to the consumer, there are a few elements in their shape which are designed for much more interesting reasons!
You may have noticed that water bottles usually have a flat bottom (with only small indents to improve the stability of the bottle). On the other hand, soft drink and soda bottles, especially those made of plastic are not flat bottomed. The foot of these bottles usually has 5 bumps (corrugation).
The bumps or the corrugations in the foot of the bottle increases the strength of the bottle. Take a sheet of paper and fold it. You can easily fold the paper. Now bend any one side of the sheet and try to fold it. You will now feel some resistance. Now, roll the paper into a pipe and try folding it. It will be a little hard to fold the paper pipe. Bending any material increases its strength and rigidity. This is because; rolling the paper increases its moment of inertia around the bending axis.
Soft drinks are served chilled. When a liquid is cooled, its volume changes and the bottle must not buckle because of this change in volume. The corner or the tip of the bump can slightly expand/contract without breaking which takes care of the change in volume of the liquid.
One more reason is that most of the soft drinks are pressurized by filling gas; the plastic bottle must be able to withstand the pressure. It should also absorb the shocks when you accidentally drop down the bottle. Next time you buy a soft drink, try crushing the bottle. You can easily crush the top part but you will find it hard to crush the foot of the bottle. This foot also improves the stability of the bottle.
The size of the bottle goes on decreasing towards the top; this is to lower the center of gravity. Doing so improves the stability of the bottle. In Cans, the bottom is arched inwards which again gives good stability to the can.
No matter how simple it may sound, there are actually quite a large number of patents and a lot of engineering behind the foot of the bottle.
