Many people simply go into a store, look for the items they want, and leave.
Others (those who are a bit savvier) go into a store, find the item they want, compare all of the sizes, and buy the one that is the cheapest per unit. These people get a lot of bang for their buck, but they also have storage space packed with stale cereal and outdated jars of spaghetti sauce (trust me, I have a neighbor who does this and I’ve watched him pitch most of a case of spaghetti sauce).
The savviest shopper goes into a store, buys small quantities of most things, and only buys the cheapest per unit on things that they love and use all the time.
Here’s why: why spend money on stuff that will sit in your cupboards for years (and often be out of date when you finally use it) when you can save that money instead? I used to buy giant boxes of cereal and such because it was the least expensive per ounce, but I would always have stale cereal at the end of the box and wouldn’t use it anyway. So why spend the extra money?
For me, these are the only things that I buy in bulk: diapers, baby food, baby wipes, toilet paper, paper towels, soap, shampoo, dishwashing detergent, laundry detergent, and laundry softener. My family uses each of these things daily or almost daily and we see no time in the near future where that will change.
Outside of these items, we buy the least expensive version of everything. I eat cereal once a week, so why buy the jumbo box of cereal when all it will do is provide some stale cereal for me in a month? We rarely eat spaghetti, so why buy the large box so that it will eat up cupboard space (and likely wind up forgotten for a long time, way in the back)?
Sometimes, I jot down the savings on each item in the store in this fashion. What did I save buying the small item in the nonessentials, and what did I save buying the best value bulk item? It’s surprising how much you can save per store visit.