He says that having the ability to tailor the amount of powder for the size of the load is one of the selling points of powder. But I'm guessing most people would much rather waste the few cents of savings in exchange for not needing to think about their dishes even that tiny bit extra.
Or just, you know, wait till the machine is filled before running it. If that takes too long you can just run a soaking cycle midway through.
amount of water used in washing cycle is +/- same, not dependent on size of load. reducing amount of powder will create weaker concentration and reduce cleaning capabilities of machine/detergent.
in reality, reducing amount of powder won't change much most of the time, because majority of cleaning happens due to physical action of (hot) water. there were multiple times when I was forgetting to put detergent and dishes went out clean. it mostly makes difference for some very dry/backed on stuff.