From the other comment by kijin - 1 liter of rice isn't actually that much (~500g, which approximates to 750 kcal). And given that most Koreans in the area didn't eat that much outside of rice (only vegetables and fruits) since access to meat was reserved for the super-wealthy, most Koreans were probably malnourished when also factoring in the manual labor they needed to do (rice farming is very labor-intensive)