What happened to the economy in buying "economy-sized"?

Mr. Aardvark described a purchase of peanuts at Target (scroll down to September 23, archives not working) that reminded me of the dozens of times these days that I find that buying a larger size does not mean paying a smaller price per unit.

In his case, he found that two jars of peanuts packaged together cost more than two same-sized jars bought separately.

Just recently, I bought razor blades. Two packs of five were cheaper than one pack of ten.

A few weeks ago, Lyman bought pasta. Two eight-ounce packages were cheaper than one 16 ounce package.

When did that happen and why? Did sellers decide that we were well enough trained to buy larger sizes that we would overlook the price differences?

If you have a clue, please leave a note in the comments.