When you are couponing you have to get away from the idea that buying the larger size will save you the most money. It may be true that it is less per ounce (which, nowadays is not always true), but with coupons you can save more by buying the smaller sizes.
Here is an example: If you buy one 32 ounce mouthwash for $5.00 and use a $1.00 off coupon you have spent $4.00 for 32 ounces. If you buy two 16 ounce mouthwashes for $5.50 and use two $1.00 coupons you have spent $3.50 for 32 ounces.
I know that it goes against everything that we were taught by our mothers and the local big box stores, but when you have coupons, buying smaller is definitely better.
This is also why you should get multiple coupon inserts each week. It allows you to use several coupons on smaller sizes in order to get the amount of product that you want.