That's only true if the bottle is fully cylindrical, whereas most soda bottles have the narrower top and further narrowing along the body.
the marginal fuel cost of shipping bottled beverages depends much more on weight than volume.
Also only true if you don't take into account the overhead of the shipping mechanism. If you're trucking these across the USA, then reducing volume by half means you need half as many trucks and a comparable savings in fuel.
From a marketing perspective, you could make a new flavor and call it Hexa-Cola. I'd try it just for the name and hexagonal bottle. (If you think that's a silly reason to drink a particular beverage, then consider this: the name is probably the only reason people drink Tentacle Grape soda. Names matter when flavors are so similar.)
As another example, many laundry detergent makers have started making "3x concentrated" detergent, which is basically the same number of loads as a regular jug but in a smaller bottle, and they advertise that part of the reason for it is that it reduces shipping costs. Now the weight change there wouldn't be as much because we're not talking about packing efficiency, but it's an example that reducing volume by itself does reduce shipping costs for these companies.
I don't know for certain that reducing the volume by 50% will guaranteed reduce fuel costs because that's a lot of math to do, but I think it's a good guess based on the above info.
With laundry detergent, they have smaller caps to fill so at least if you pay attention then you won't over-use it.
I've also noticed that a lot of food product vendors are turning the old 'more is cheaper' paradigm upside down. I keep running into products where the 24oz container is more expensive than two of the 12oz containers, etc. It's just like the extra large pizza scams.
Coca-Cola spent millions of dollars on optimizing this in the 90s. I would not care to bet that on casual inspection you've noticed something obvious that they missed.
We had fun throwing 2L bottles on the floor. If they struck the floor smack on the bottom of the bottle they would bounce back up. The impact increases the pressure inside the bottle. We'd do this a couple of times, then the last drop would be top down, hopefully breaking the cap launching a coke powered rocket to the roof! That's the quickest way to break the bottle without using tools I think.