We are conditioned in our culture to lean towards "the more the merrier" (for just about everything except family size!). I've found that, even with a large family, there are some things that are nice to buy in smaller sizes for various reasons, even if they cost a little bit more:
Condiments - a small bottle of mustard is about 88 cents. It lasts about three months in our family. Having a smaller size is easier for the kids to use and takes less space in the fridge. A squeeze bottle of mayo prevents my little ones from contaminating it with tuna or pickle relish and if they accidentally leave the bottle out on a warm day, there's a lot less waste. (You can also keep a larger jar in the back of the fridge for refills.
Ice Cream - Unless we're having a big party, I avoid the huge buckets of ice cream. Sure they're less cost per ounce but I don't really need those extra calories. We prefer to get the really good ice cream when it's on sale and just enjoy it for a short time.
Soda for sickness - The Seven Up or Sprite we keep on hand for illnesses are best in cans rather than 2-liter bottles. That way you can store the extra cans until needed instead of feeling like you have to use up a whole bottle. I especially like the "mini" Sprite cans.
Well, you get the idea for a few food examples. Are there any things for which you've found that "less is more"?