Best cooling results are achieved when the air is free to circulate around items stored in the refrigerator or freezer. When items are packed closely together, the air cools the outermost items but the items in the center will not be thoroughly cooled; at least not quickly. Cool air never reaches the center items and they cannot be properly chilled.
Make sure that air can flow over and between the shelves as well as over the items stored there. Overfilling the refrigerator or freezer will result in inefficient and inadequate cooling.
An underfilled refrigerator also will not cool as effectively. The things you store in the refrigerator and freezer have thermal mass. Once they cool down, they have a tendency to stay cool. When you open the door, the cold air spills out and is replaced with room temperature air. However, all the food remains cold. Because the food takes up space that would otherwise be filled with air, there is less work to be done to cool the refrigerator back to its storage tmeperature. The more items stored in the refrigerator, the less air space that must be chilled after each time the door is opened. This is true up to the point where there is not enough space for air to flow.