Packaging Digest’s recent article on Slack-fill, missed the point. They linked slack-fill to sustainability. Certainly if a package is made smaller, there is less packaging material. Marketing will choose product sizes to satisfy their customers, but choosing the container’s physical size is constrained by both the laws of physics and regulations.
As in the picture, you can certainly put 3 oz in a 4 oz container, but if you purposefully include “empty space” you are misleading.
The FDA considers Slack-Fill — MISBRANDING.
a food shall be deemed to be misbranded if its container is so made, formed, or filled as to be misleading.” 21CFR100.100
The Federal Registers says “Slack-fill is the difference between the actual capacity of a container and the volume of product contained therein. Nonfunctional slack-fill is the empty space in a package that is filled to less than its capacity.” There are some reasons why you can have “space” such as to accommodate packing equipment, unavoidable settling, and to protect contents.
In short, sometimes things we don’t even see, like ’empty space’ can be important. Make sure your packages are appropriately filled with product.
Source: Packaging Digest – Can slack-fill law drive sustainability in packaging?