I think your comparison to other subs is off thought. Depending on the meat of the sub you might be reducing protein, but the Norwegian sandwich definitely has less fat (less cheese, less calories in the butter than mayo or other sauce, less fatty meats). Usually the bread preferred also has more whole grain and fibers than what's used in the US.
Butter is much higher in saturated fat and calories than mayo, and you're putting butter on 3 slices whereas you usually only but mayo on one (per the article). One tablespoon of butter is 100 calories and 7 grams of saturated fat, one tablespoon of mayo has 60 calories and 1.5 grams of saturated fat.
If I look at the nutritional information for Kalles Kavier (which someone suggested as a topping), it seems to also be pretty high in calories with a lot coming from extra carbs (sugar).
A Norwegian "sandwich" has one slice of bread. The picture at the top of the article shows 3 stacked open-face sandwiches.
Sometimes people will do "British-style" sandwiches when putting on things like jam to avoid making a mess, but open-face sandwiches is most traditional.
> last year I have made a matpakke with 3 slices of bread
Which also aligns with what's said in the article. If you eat three of these in a meal, you're getting one more slice of bread than you'd get from a meal consisting of a single sandwich.
> Compared to other sandwiches the Norwegian sandwich seems to substitute protein (less meat and cheese) for more saturated fat and carbs (an extra slice of bread, butter on the bread).
It's meaningless to compare to a "single sandwich" anyway, because e.g. the bread size will vary dramatically depending on country. I live in the UK now, and the slices used for sandwiches here are often 50% or more larger than what I'm used to from Norway.
But especially if talking about the amount of protein relative to carbs or fats, I don't think it's very different - each of those three slices of bread gets their own slice of meat or cheese. And not by any means everyone will use butter, while you'll get plenty of sandwiches elsewhere (e.g. UK) that are buttered.
A lot of people seem to be under the wrong impression that simple foods are low in calories. Compared to other sandwiches the Norwegian sandwich seems to substitute protein (less meat and cheese) for more saturated fat and carbs (an extra slice of bread, butter on the bread).