The broth is the surprise here. Nam sai kuay jab is a clear pork stock loaded with white pepper, and it comes in at about 25 calories per bowl. It is one of the most warming things you can eat in Bangkok for almost nothing. The rolled noodle sheets are 100g and about 110 calories, so before any topping goes in you are at 135. That is why the plain hdmall figure of 240 is so low: it is noodles, broth and a modest amount of offal.
The toppings are where a Chinatown bowl earns its 350. Pork intestine, liver and a piece of blood jelly are about 80 calories for 50g, and they are decent protein. Then the moo krob: a standard scatter is 30g and about 110 calories, more than the whole broth and offal put together. A piset bowl with extra crispy pork and a boiled egg lands around 480.
Nam khon (the dark version) is a different animal. The broth is thickened with tapioca starch and flavoured with five spice and soy, and the noodles are usually a bit more, so a bowl runs 380 to 420 even before crispy pork. If you want the lightest kuay jab, it is nam sai, offal only, no moo krob, at about 240.