Khao soi is a curry and a noodle soup and a plate of fried snacks all in one bowl, which is why it is 700 calories when a normal Thai noodle soup is 300. The broth is coconut milk with red curry paste, about 250 calories for 200ml. The soft egg noodles underneath are 210 for 150g. Then the handful of deep fried noodles on top is another 100, and they are the part everyone loves.
The published numbers disagree more than usual on this one. Some Thai calorie tables list khao soi at around 400 per cup, which describes a small bowl with thin broth. Chiang Mai shops serve big bowls with thick coconut broth and a whole drumstick, and 700 is closer to what lands on the table. Beef khao soi with a chunk of braised shin is heavier still, around 800.
The sides are the redeeming feature. Pickled mustard greens, raw shallot, lime and chili paste are all close to zero, and they cut through the fat. The bowl has 30g of protein, so it is a complete meal. It is just not a light one, and a second bowl, which is a normal thing to do in Chiang Mai, is a 1,400 calorie lunch.