Bigger may not always be cheaper.
For the most part, a 2-liter bottle of soda pop may cost less than even a 20 or 24-ounce bottle.
Watch ads. Look for deals. Many times, I've seen 2-liters on sale for $.99 when a 24-ounce is $1.59.
Also, even though the generic store-brand may not 100% taste like the real thing--cola may taste similar to Coke or Pepsi--but when your budget is tight and you're getting out of debt, settle for the cheaper brand.