Cart noodles are the only Hong Kong meal where you set the calorie count yourself at the counter. The noodle choice alone swings 100 calories: rice vermicelli is about 175 for a serving, oil noodles 220, instant noodles (出前一丁) 280 and up. Then the soup base adds anywhere from 40 for clear broth to 130 for satay. Before a single topping goes in, the bowl is somewhere between 215 and 410.
Toppings are where people lose track. Pork skin looks harmless but it is about 230 calories a portion because it is fried before it is braised. Beef brisket is 200, sausage 210, fried fish skin 140, pork intestine 180. On the other side, pork blood is 65, braised tofu 75, fish balls 95, white radish under 30. Three heavy toppings versus three light ones is a 400 calorie difference in the same bowl.
The 570 calorie bowl above is what most people actually order: oil noodles, brisket, something cheap like pork blood, a green, curry soup. It is a decent meal with 28g of protein. The danger bowl is the one built by hunger at 11pm in Sham Shui Po: instant noodles, satay soup, pork skin, sausage, fried bean curd. That is 850 calories and around 45g of fat, with less protein than the lighter bowl.