Xiao long bao feels light because the skin is so thin and the whole thing is one elegant bite bro. But that soup inside is not water, it is rich pork broth set with gelatin and fat, and the filling is fatty pork mince. A basket of 5 is around 320 calories. Not crazy, but more than the delicate look suggests.
It is still a decent order. You get 15g of protein from the pork, it is steamed, and it is genuinely satisfying because the hot broth makes each one feel substantial. Just do not treat a couple of baskets as a light snack. Two baskets is 640 calories, which is a full meal, soup dumplings or not.