Perhaps the population grew when they started using chopsticks was because it was more hygienic. Assuming that people ate with they're hands before, think of how dirty it would be especially after a long day of hard work. The grim gets on their food, which enters their system, and before long, they fall ill.
Now, I suggest you fact check this because this is all based on my own assumptions. I'm not too familiar with the Zhou Dynasty, so please take what I say with a grain of salt. But I do hope this has nudged you in the right direction!
