I know it's tough. It gets more expensive when you buy smaller portions of some items. However, when it comes to things like lunch meat, you can get smaller portions at the deli counter without having waste or spoilage before you can eat the family pack. Also, one tip would be to buy the larger portions and use the foods to make meals you can freeze for future eating. I love shopping at Costco, however some of the products are way too big for just my husband and I, so I often ask family or friends or neighbors if they would be interested in sharing. So that's another option. Also, if you don't mind eating the same thing for dinner or lunches for a few days, you can cook with the larger amounts of food and have leftovers.
Did you know that you can freeze milk? My mom does it all the time. She buys the gallons when they are on sale, pours it into smaller containers and freezes it. When she needs milk, she takes a container out of the freezer and puts it in the fridge to defrost for about a day. The taste isn't affected at all (she says).
Hope some of this helps.