This is a shipping industry term for cargo that takes up a lot of space but is very light. Shortly, balloon freight is low-weight, high-volume cargo.
Think of shipping a truckload of cotton balls, styrofoam coolers, or empty plastic jugs. The items will fill the entire trailer long before the truck gets anywhere near its legal weight limit.
This type of freight is named balloon freight because, like a balloon, it is mostly air and takes up a large amount of space (volume) relative to its minimal weight.
For a shipping company, balloon freight presents a specific challenge. Carriers charge based on both weight and space. Since the truck fills up by space rather than weight, the carrier has to charge based on the volume the shipment occupies, not its actual low weight.
This is often calculated using a formula called dimensional weight (DIM weight) to ensure the carrier is fairly compensated for the space their cargo uses.