Portion size and cleanliness doesn't matter if the food is lacking nutrients and contains toxic chemicals.Very hard to know the reasons for longevity in a country. Culture, food, physical environment, healthcare, etc.
As for food, my personal guess would be a combo of portion size, cleanliness, food quality, etc. Not one element. And they seem to use a surprising amount of preservatives for a country that is very concerned with freshness.
So yes it is mainly the way the food is produced. Nothing else. Because our bodies work on a biological factor, so less toxins and more nutrients = healthier body
simple as that. cleanliness wont have as much of a factor as you think, maybe it is involved in how often you are sick.
And portion size will mainly affect your weight rather than health which weight can affect health yes, but in a different way than we are talking. So no. I am sticking by the fact that it is Purely in how their food is produced from crop health to chemicals safety and amount.