Which gives us 480,000 ounces of coke in 1950 vs 505,000 ounces of coke in 2015. A 5.2% increase in the amount of coke you can buy for a year's salary. (I'm actually kinda surprised the price is that stable.)
It's easier to sell one bigger package than several smaller.
I would not be surprised, if the 505,000 ounces in 20oz packages cost less to produce in 1950 prices, than 480,000 ounces in 6oz packages. (Just one physical package, not three separate).
It's a set of tradeoffs. The 6oz bottles were (I think) returnable. Even better than recycling, they only needed to be washed/sterilized to re-use. I'm old enough to remember Coke and other brands in returnable bottles up to 16oz.
They were heavy. The transportation costs savings with the lighter plastic bottles (combined with maybe the raw materials savings, and savings not having to transport, handle, wash, and sterilize the emptys) made plastic more economical than returnable glass.
Manufacturing cost for glass bottles almost certainly higher than plastic. You have to melt either sand or recycled crushed glass which takes a lot of energy.
Some jurisdictions do have returnable plastic bottles. They are much heavier than the disposable ones but still lighter than glass.
Interestingly for some reason beer in plastic bottles has never caught on, it's mostly sold in glass bottles and cans.
I wouldn't be shocked if they make sure it's always about as affordable as it used to be, since they do want everyone drinking it (as opposed to companies like Apple going for status/luxury bucks).
Lesser quality ingredients are also used. Additionally, Coca-cola is mostly water, which means its price will largely be driven by the price of water, which hopefully means that everyone will still be able to drink it.