Miso ramen is 621 calories because everything in it is bigger. The noodles are thick Sapporo style and the standard serving is about 230g cooked, around 340 calories. The broth is miso paste blended into pork stock and finished with lard, about 130 calories for 350ml. Then the wok topping of bean sprouts, cabbage and ground pork is fried in oil, another 60 to 70. Compare that with a shoyu bowl where the broth is 40 calories and you can see where the extra 200 comes from.
The Sapporo classic adds butter and corn. A 10g pat of butter is 75 calories and a spoon of sweet corn is 40, so the famous 'miso butter corn ramen' is about 735 calories for around 1,100 yen. It is still a normal meal, but it is close to a McDonald's Big Mac set. Order it once in winter, not every Friday.
The bowl is 839g, which is why it fills you up for hours. About 400g of that is broth. If you eat the noodles and toppings and leave the soup, you drop to around 500 calories and cut the salt from roughly 6g to about 3g. The miso broth tastes good enough that people finish it, so decide before you start.