The dough is where the calories are, and it is more dough than you think. A street banh bao weighs 140 to 160g and roughly 90g of that is steamed wheat dough made with sugar and a little oil, about 240 calories per 100g cooked. So 220 from the bun alone before you find the filling. The pork filling is 35g of minced shoulder with onion and wood ear, around 85. Two quail eggs are 30 and the slice of lap xuong is 25. That is your 360.
It looks like a snack and eats like a meal. At 360 calories and only 14g of protein, a banh bao is more than a plate of banh cuon (300) and close to a bowl of pho ga (400), but nowhere near as filling, because 150g of bread does not sit in the stomach the way 500g of soup does. The usual move is a banh bao and a ca phe sua da for breakfast, about 490 together, and hunger by 10am.
The variants spread wide. Long Chau lists banh bao nhan dau xanh (mung bean) at 150, banh bao trung muoi (salted egg) at 150 for a smaller bun, banh bao khong nhan chien (fried plain) at 180, and the big pork and 2 egg version at 350 to 400. The chain shop ones, Tho Phat style, run 180 to 200g and 400 plus. A banh bao chien, deep fried instead of steamed, adds 80 to 100 of oil to whichever version you pick.