The number surprises people because tonkotsu looks heavy. It is the cloudiest, fattiest looking broth on the menu, yet Ichiran's bowl comes in at 525 calories, lower than a miso ramen at 621. The reason is portion size. Hakata style noodles are thin and the standard serving is only about 110g cooked, roughly 236 calories worth, which is why the shop lets you order kaedama refills for 210 yen. The broth itself is where the fat lives, around 150 calories for 300ml, but you are not eating a giant pile of noodles with it.
The trap is the refill culture. One kaedama is 236 calories. Two kaedama and you are at 997 calories for what still feels like one bowl of ramen. Add a marinated egg for 80 calories and extra chashu for another 150 and a single Ichiran visit lands at 1,200 calories, which is more than half of most people's daily target. The bowl costs about 1,080 yen, and every add-on is cheap, so the bill stays small while the calories do not.
Drinking the soup matters more here than with shoyu ramen. A shoyu broth is mostly water and salt. A tonkotsu broth is emulsified pork fat, which is why it is white. If you finish the bowl to the bottom you get the full 150 or so calories of broth, plus about 5g of salt. Leaving half the soup saves around 75 calories and 2.5g of salt, and most Japanese ramen eaters leave it anyway.