Why are Korean people so skinny? Is it because of the food they eat?
So many Koreans are "not skinny," so your argument is invalid, lol. It could be a combination of many things such as genetics, food, and lifestyles. Young Korean women are very conscious about being fit. Many go on a diet immediately once they gain a few pounds.
But food may play a key role as well. A Korean woman I know once told me her story. When she was in college, she ate bread every single day because of her work. She wasn't forced to eat bread or anything. But she loved bread so much she ate a lot. (It didn't cost her anything.) She gained so much weight that nobody could recognize her anymore. After she quit her job, she went back to "normal." She weighs around 38 kg (85 pounds) now.