Or proportional to the household eating habits. I'm definitely in the upper income bracket of my son's state primary school and he brings a sandwich and a piece of fruit like every other kid because it's a sensible amount of food for a small child.
a sandwich is enough for a mid morning snack, but not for a midday meal unless the kids get another meal right when they come home. traditional wisdom has it that you should have a good breakfast and lunch, and a smaller meal for dinner. that's possible when the kids come home no later than 2pm but if they usually come home at 3-4pm or later then a sandwich is not enough to get them through the day.
the official US guidelines for school provided lunch range from 550–850. that's a bit lower than i would choose based on daily needs especially for older kids. breakfast and lunch should be more than 1/3rd each. dinner should be less than 1/3rd.
i am from austria. a sandwich here is considered a snack. not a meal.
This could also be an overseas perspective because in my opinion (having just returned from USA) the single meal portions are very large in USA compared with overseas.