I'll never quite understand lunch culture in Japan.
If a meal is composed of a single sandwich instead of an appetizer, two dishes and dessert. That's not a meal, that's a snack!
Its actually a balance thing.
In japanese its called 一汁三菜 (Ichijuu Sansai)
Its comprised of one soup, 3 sides and rice.
Usually served with some kind of pickled vegetable as a pallete cleanser.
弁当 (bento) follows the same sort of principles if balanced meal. Bento usually follows a 3 - 1 - 2 rule
3 parts carbohydrates 主食 (Shushoku)
1 part protein 主菜 (Shusai)
2 part vegetable 副菜 (Fukusai)
Its all about having a balance of nutrition, and flavor.
A lot of japan also follows a 350g of vegetables rule. To the point that it is actually labeled on the foods in like a convenience store.