Which weighs more: a pound of lead, or a pound of gold?
That's a standard “trick question”, sometimes used in bar bets and the like. One wants to say that they're, of course, the same: a pound is a pound. But then the trickster will tell you that lead is measured in the avoirdupois system, where a pound is about 454 grams, while gold is measured in the troy system, used for weighing precious metals, where a pound is about 373 grams. So a pound of lead is, by that reckoning, more than 20% heavier than a pound of gold. OK, changing units of measure in the middle is bogus... but all's fair in bar bets.
Which weighs more: a pound of potatoes or a pound of coffee?
If you get your coffee in “one-pound cans”, the answer is that a pound of potatoes is heavier — about 23% heavier, currently.
If you're selling coffee in one-pound — 16 ounce — cans, and you have to increase prices by 10%, well, you could just increase the prices by 10%. But that tends to get people angry when it's done all at once, and too many price increases in a short time isn't good either. But if you just quietly start selling only 15 ounces per can, then you can hike the price per can by only about 3% and get the same effect. And you can separate the events — switch to 15 ounces first, and a few months later take your 3% price increase. The thing is, few will notice that they're getting less product. Sure, it's printed on the can, but most people don't read that, and the few who do won't be loud enough to piss off the bulk of the consumer population.
And that's what sellers of mass-market coffee did, several years ago. They've since cut the amount down to 14 ounces, and finally, now, to 13, using slight changes to the can shape to hide the difference (some brands, such as Folgers, switched to plastic “cans”, further disguising the change). And it's not just coffee, of course: the “quart” jar of mayonnaise that I have in the ’fridge is only 30 ounces, 6.25% less than a quart. With glass bottles, they'll often increase the indentation of the bottom. It's perfectly legal, naturally.
Sometimes, though, the manufacturers will actually start giving you more product, rather than less. They'll tout this change loudly, of course, rather than trying to sneak it past as they do when they shrink things. More product, hey, gotta be good, right?
Because, you see, most people aren't good with percentages and decimal points. Suppose they're selling 30 ounces of product for $1.39. Maybe they'll put “Now! 10% more product!” on the package, and sell you 33 ounces for $1.59. Well, OK, 33 ounces is 10% more than 30... but 10% more than $1.39 is $1.53, so that extra 6 cents is a price hike that's hard for most people to see. In this example the difference is very small, but the whole thing has set them up for a larger hike in the future. And they're also using the change as a marketing ploy, pretending that it's a benefit to the consumer.
An interesting variation on this is when they increase the package size out of proportion to the increase in product size, making you think there's more to it than there is, which lets them get away with a larger price increase. 40% more product with a 60% larger package — and 45% higher price — can easily hide the effective 3.5% price increase and make you think you're getting more value than ever before.
Perhaps the most insidious one, though, is the truly deceptive packaging. My favourite example is Andes candies (which I never knew were associated with Tootsie Roll until I wrote this). Look at the main package, the large-ish retail box at the top (referring to the image at the time I'm writing this, of course). When you open the box, you find that there's a lot of cardboard in it, and the actual product only takes up a smaller rectangle in the center. Everything around it is air and cardboard. They used to have a window in the middle of the box that showed you the active rectangle and let you infer that the rest of the box contained product too. According to one estimate, reflecting the size of the box at that time, the box appears to hold almost 60% more product than it actually does.
Of course, they'll say they're not deceiving anyone: the quantity printed on the box accurately reflects what's inside. That's true, but as I said above, most people don't pay very much attention to that; they assume that the box is mostly full, rather than having a significant portion empty. There are certainly cases where it makes sense to have protective packaging around the product, but this is excessive by any judgment, and can have no purpose other than deception.