Flan is eggs, milk and sugar, and there is no fat added beyond the yolks. A cup uses about 1 egg, 60ml of milk and 15g of sugar in the custard, so 90 calories, plus a 12g caramel layer that is pure sugar at 30. Long Chau's 145 per 100g is exactly what that adds up to. Compared with the other things on a che cart, a 125 calorie flan is about a third of a che ba mau (330).
The size of the cup decides everything. Long Chau's 65 to 70 per piece refers to the small 40 to 50g flans from supermarket packs and carts that sell 3 for 20,000 dong. Nobody eats one of those; three is 200. The big 100g quan version with a thick caramel base is 145, and banh flan pho mai (cheese flan) is 200 per 100g because of the cream cheese folded in. Flan with condensed milk drizzled on top, which some Saigon carts do, is 160 to 170.
It is one of the few desserts with real protein, about 4g per cup from the egg and milk, and the caramel gives you the sweet hit at 30 calories rather than the 90 of coconut milk on a che. A flan and a ca phe den after dinner is 130. A flan and a ca phe sua da is 255. The Hanoi caramen version served with sua chua (yogurt) and coffee jelly is 220 to 250 for a bigger bowl.