Here is a site that everyone should visit. Consumer Psychology is a site dedicated to the analysis of the way people shop and consume products.

Do you think you are saving money by buying the bulk value packs? No way, they are probably costing you money.

Larger package size increased product usage between 7% and 43%.

Do you think you are saving money by buying "no name" or generic brands. Nope your not.

Consumers poured 9% to 36% more from a sale-priced bottle of liquid cleaner (lower per-unit cost) than a full price bottle.

Why do you think stores impose a "Limit of 12 per customer", you buy more when they do.

When consumers shop for groceries, such as soup, they determine how much to buy based on how many they normally buy. For example, a customer might normally buy 2 or 3 cans of soup without giving it any thought. In effect, they "anchor" on the number 2 or 3 when they decide how many to buy. If you could increase that anchor number through marketing, purchase amounts would rise as well.

This site makes fascinating reading.