No, 77 is pretty explicitly mass based - yes, it’s mass displacement, but page 17 “all non starships consume fuel at a rate of 10 kilograms (1/100th of a ton)...”, while page 43 provides guidelines for determining how much of an item is present in a ton of its matching trade good, and also makes it clear that trade goods are measured by weight. 

But on your other point, cargo sections are tiny. A fully loaded 40’ container has an average density of about 350kg/m3, which is honestly about half the density of wheat grain. volumetric ship design gives us literal dTons of waste space if we try and reconcile mass based performance with current designs.