It looks like they might be taking advantage of the customers who like to carry the smaller size around with them for convenience. The 2 liter size is usually only used at home by the family. Just one way for them to make money while shortchanging the consumer. I see a lot of twin packs of things that look like it's in sale packaging, but many times they cost more than buying the item individually. I found that to be true with toothpaste.
I always assume that it is because the cost of processing, packaging, etc... is about the same for both the small and large package. So in some cases the larger item comes in cheaper or at about the same price. As I get older I find myself buying more and more single serving items even though they are more expensive to purchase they work out to be cheaper options for me in the long run. Things like a large can of V8 juice vs single serving cans or single serve applesauce and fruit snack packs vs a family size bottle or can. Sometimes you need to look beyond the price to get the best deal for your situation.
I agree Bea, sometimes we buy the tiny V-8 juices, simply because opening a large one usually means it goes to waste eventually, with just two in the household. Been buying our applesauce and pudding that way too, single servings are more fresh and handy.