One of these things is not like the others...
Buying big soft drink bottles instead of small is both cost-effective (as they're usually cheaper per liter) and a plastic saving measure.
(What I particularly dislike about modern consumerism is the infatuation with small: small bags of chips, small bags of fruits, individually-packaged sausages, small bottles of drinks, small bottles/bags of cosmetics, etc. Combined with disposability of packaging, it's just a huge generator of plastic waste.)
Edit: I agree with your complaint about packaging, but it's another problem that's not uncompatible
True, but bigger serving sizes have been shown to increase consumption. If you reduce soft-drink serving sizes from 500ml to 350ml, for example, most people will not purchase two - and will consume less overall.
Carbonated drinks fizzle out quickly and they don't taste that good afterwards. So, if you buy bigger bottles, you are inclined to drink more so it doesn't go to waste.
No surprise they have an obesity problem. Of course Americans are also usually nice people in a big way, let's not forget.