When you purchase a box of cereal or bag of potato chips at the store, you often come home to find that 2/3 of the container is filled with air. This is because although the container may be filled up during production, while the container is "in transit", all the contents are slowly settling to the bottom.
There is a scripture I'm looking for that makes a similar point. Speaking of God's generosity, makes an illustration of how not only does God fill your bag of wheat or grain to the brim, but he "pushes down" on the bag, compressing the contents, so he can fill it even more.
Can anyone recall which scripture I am referring to?