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.