I see your point about hauling and in that reguard it does affect how many items a job should produce, but it has little effect on any discussion about volume.
Well, the number of units for construction must go up if there is to be greater diversity in construction materials. Right now, to build a wall you need one stone, one log or one block.
If instead the wall required several units of material it would be easy to require several different types (like bricks and mortar or beams and boards). The size of the items (and the wall) is not really that important, since production and use can be scaled up together (e.g. mining would produce up to three stones per square dug, but wall building would require three items worth of stone).
To take it to the logical extreme, I think it would neat to model each individual brick or stone in a wall (which could number in the hundreds). This would allow interesting things like using a single brick as an improvised weapon. However, unless hauling, production and construction are dramatically overhauled to deal with the large numbers of items this would imply there will need to be at least a bit of abstraction. That's why, in my original example, I said you would need several "pallets" of bricks, each of which would be treated as a single unit. By using a somewhat vague unit size Toady would also be able to put off the volume question more easily (there would be no need to say how many bricks are in a pallet, or exactly what the dimensions of a beam are).
I think that the hauling improvements will still need to get done before any construction material diversity is implemented. Even multiplying the number of hauling jobs by a relatively small number like 3 or 4 would badly slow down the progress of any large construction project.