The rice does most of the work. 100g of cooked glutinous rice is about 180 calories, and it is fried in lard with shallots before it is wrapped, which adds about 40 more. The nonya filling is lean minced pork with candied winter melon and mushroom, about 130 for the lot. So 352 for a dumpling that fits in one hand. It is the same calories as a plate of white carrot cake, and people eat it as a mid afternoon snack.
The Hokkien style is heavier. It uses pork belly instead of lean pork, a whole salted egg yolk, a chestnut and sometimes dried scallop, and the rice is darker because it is fried with more soy and oil. That dumpling is about 450, and the big ones sold for the festival can pass 550. The kee chang, the plain alkaline dumpling with no filling that you dip in gula melaka, is about 120 without the syrup.
It is also dense in a way that does not register. Glutinous rice is packed tight, so a 150g bak chang has more rice than a 150g scoop of ordinary rice. That is why it sits in your stomach for hours. That is also why one is enough. Two nonya chang is 700 calories, which is a full dinner.