There are many considerations when selling a product. One of the important ones is the design of the packaging.
In recent years soft drinks have been offered in Canada in cans like those above. These squat little things contain 237 millilitres of product each.
When I went to buy some groceries yesterday I decided to buy some Coke Zero. When I came to the pop isle I found that the small can format had changed to the one above, as it has for other Coke brand products. It's taller than the previous format, which was the same 237 millilitre package as the Pepsi cans above. But if you look at the picture carefully you'll see that the can actually contains less than the old version. It contains 222 millilitres of beverage, 15 less than the squatter format.
I doubt the new format was intended solely to reduce the amount of product. Given that part of the reason for his size of pop is to appeal to those on diets it wouldn't surprise me if they hope to encourage purchases by making people subconciously associate drinking that size with getting slimmer. They might also be hoping to attract customers who like energy drinks like Red Bull, which come in similar slim cans. But whatever the other reasons they've also apparently decided to use the change to increase their profit margin. I have no idea how much 15 millilitres of Coke Zero or any other soft drink costs to make. But if you make a tenth of a cent or whatever per can versus the 237 ml size while selling it at the same price it will quickly add up.