Life is rarely what it seems. The packaging of goods is certainly no exception to this rule. In fact, getting what you see is even rarer than you think.
The word "nominal" comes from the Latin "nominalis" and "nominis" ("nomen"), meaning "pertaining to a name or noun." The word "volume" comes from the Latin "voluminis" ("volumen"), meaning "a roll, scroll or coil (of manuscript)." The sense of volume as a quantity or bulk comes from the ancient perception of books as large and heavy.
Merriam-Webster Online defines the phrase "nominal volume" as the quantity of a substance (usually a liquid measured in cubic units---quarts or litres) that a pre-measured and prepackaged container is said to hold. This amount is a general, identifying approximation and often differs slightly from actuality.
Ideal vs. Reality
The concept of nominal volume (or more generally nominal size or value) focuses on the difference between what something is "in name" (ideally or in theory) and what it is in physical actuality. A nominal value is like an archetype---the ideal or most common (standard) representation of something for the purposes of easy identification.