OK, the main filler in regular detergents, both then and now, wasn't wood pulp but sodium sulfate. Sodium sulfate's "benefit" is mainly that it's cheap. It adds little cleaning power to the detergent, although it's mildly alkaline and so might aid detergency a bit. It is added to help with "dispensing". That is, it tends to flow better than the other ingredients in a powder, and helps to keep them from clumping. But it also makes a 16 load box look huge, and I'm sure the marketing/sales geeks observed that sales and profits increased the bigger the box.
Wood pulp is added to detergents, modified in the form of carboxymethylcellulose ("CMC"). It functions to help keep dirt released from soiled fabrics from redepositing. In this regard it tries to replace one of the functions of complex phosphates. I doubt that CMC adds much bulk to a powdered detergent in the amounts it's probably added, but I may be wrong there. I have used a purified form in the lab and the dry stuff is really difficult to get wet at first, but in time it absorbs water and in large concentration makes a saline solution kind of viscous.
The creation of detergents (both then and now) from petroleum products doesn't really add any solvents to the mix. It's simply a quick and cheap way to make a synthetic surfactant, and as we all know surfactant is by volume a small fraction of a typical laundry detergent, the bulk being taken up by breakers, water softeners, and fillers.
My take on the mfg's dosing instructions is that these amounts were geared towards truly filthy laundry with hard water. I know from my own experience that when I wash very dirty garments, I have to use 2 to 3 times as much detergent (plus additional STPP) to get the grime out, and sudsing is adequately suppressed by the oils, greases, and soils on the laundry. No doubt mfg's also liked to see homemakers add more of the product than was necessary, but I think most of us would tend to observe the suds level and adjust the detergent added accordingly. One big benefit of a traditional top loader is that you can readily see the sudsing and also add more detergent if needed.
And, as far back as I can remember, most vintage detergent boxes included this qualifier: "Add more detergent for large, heavily soiled loads, and less detergent for small, lightly soild loads".