About 150g of the roll is rice, and that rice is seasoned with sesame oil and salt before it goes anywhere near the seaweed. That is 220 calories from rice plus 45 from the sesame oil, so 265 of the 400 is there before a single filling. The egg strip, ham and crab stick together are about 95. The pickled radish, carrot and spinach are 20.
Fillings change the number a lot. Tuna kimbap is tuna mixed with mayonnaise, and the mayo alone is 90 to 100 calories, which is why Kimbap Cheonguk lists it at 570 for a 300g roll. Cheese kimbap adds a 60 calorie slice. Bulgogi kimbap adds sweet marinated beef for about 150. Vegetable kimbap with no ham or egg is the low end at 355.
The trap is the second roll. At 3,500 to 4,500 won a roll it is cheap enough that two rolls with a cup of tteokbokki is a normal order, and that is 1,170 calories for a snack that felt light because it was cold and cut into pieces. One roll and a bowl of eomuk broth is 500 and does the same job.