The 2:1 ratio is important as properly proportioning your cooler's space is directly related to your cooler's performance. The 2:1 ice-to-food ratio is the optimal proportion for cooler performance, such as getting a 50 qts cooler over a smaller cooler. The extra ice acts as a buffer for additional cold retention for the cooler, but only to a certain point. Any more, and you'll get diminishing returns. Any excess air can expedite melting as external temperatures can heat it. So fill up those extra empty pockets with ice cubes. You can trap the cold air with a damp towel or a reusable freezer sheet. The sheets and towels also inversely prevent the warm air from getting inside.