The attitude is strange with vending machines and other cultures. I saw a vending machine selling 20oz and 1.5l (I think?) bottles of coke for the same price. In America, that would be unheard of, why would anyone buy the 20oz bottle. But in Japan, it seemed like many people just bought what they needed.
It's common for American grocery stores to sell 20 oz bottles for +-20% of the price of 2 liter bottles. The differentiation here is entirely in perception. The smaller bottle is usually chilled and seems like an individual serving. The larger bottle is considered to be a bulk purchase. Often, consumers will purchase both in the same trip, intentionally oblivious to the value differential.
Maybe it's more of a placebo; you're more likely to drink the 20oz bottle quicker than the 2-liter. An open (even re-capped) 2-liter bottle will lose carbonation over N days quicker than a 20oz you drink in maybe an hour or whatever (at least that's how I see it)
In America, when I ask for half a cup of coffee at Starbucks (no I don't ask for a discount), because I don't want/need that much coffee, I get weird looks.
Or when I am at Taco Bell and I just want a small cup of soda, instead of that giant cup they give, I always get the "are you sure?"
I'm not even Japanese and I'd buy the small can. I have no use for 0.5l of coke, let alone 1.5l. It's supposed to make me a bit more concentrated, not replace my water.
You probably have to bring the trash with you for a while, so ending up with a smaller empty bottle can be preferable. There aren't a lot of public trash cans.
(Before I realized this, I was once perplexed by where to throw out a food wrapper. I went into a restroom in the subway station and threw it out in the only receptacle I saw, which turned out to be the hand dryer. That was embarrassing.)
