Bun bo Hue is the heaviest of the big Vietnamese noodle soups, and three things drive the 630 calories. The noodles are thicker and the portion is bigger than pho, so 220g of bun is 240 calories. The gio heo, that slice of pork hock with the skin and fat attached, is 200 calories for an 80g piece. And the broth is finished with sate, chili fried in oil, which is 45 calories per tablespoon and every bowl gets at least one.
The beef itself is the healthy part. A 60g portion of sliced shank is about 100 calories and mostly protein. The cha cua (crab and pork paste) cube is 60, and the huyet (blood cubes) are almost nothing, maybe 20 for a couple of pieces. So if you removed the pork hock and skipped the chili oil, the bowl would be closer to 400.
A bowl dac biet at a good Hue quan comes with everything: shank, hock, blood, cha cua, sometimes a piece of beef tendon, and a second spoon of sate. That is 750 to 800 calories, and the broth is salty enough that you will drink a lot of it. The 45,000 dong price makes it feel like a light meal. It is not.