Morality is a mutually agreed behavioral rule set that arises among groups to assign immediate personal benefits and costs to positive and negative externalities. Specific rules, then, are adapted to the specific externalities in a given group in response to awareness of the externalities.
Examples. Random killing always has negative externalities for all groups, so murder remains universally wrong. But when penicillin, birth control and welfare are invented, the externalities of STDs and starving unwed mothers are reduced, so premarital sex becomes de facto acceptable after millenia of taboo.
Obesity has both a personal externality (you live poorly and die early), as well as a public externality (health costs). A newly developed country like South Korea gains awareness of this only over time, so the moral rules don’t respond immediately. So one would expect a brief explosion of obesity, followed by a recovery.
But wait a minute. For decades, the US has enjoyed a diet rich in fat and sugar, and practically unlimited in quantity. Beginning about 40 years ago, 3 simple rules (eat only when hungry, no sugar, no saturated fat) became widely known and publicly promoted. So why did the obesity epidemic began only 15 years ago? Are we becoming immoral?
Possibly, but there is another potential explanation: some new cause of obesity, for which there is low awareness.