Fish balls are the cheapest calories on the street in the good sense. Six balls without sauce are about 76 calories, which is less than a single egg tart (195). The Centre for Health Protection lists them at the very bottom of its street snack ranking, under stinky tofu (174 a piece), fried intestine (160 a skewer) and egg waffle (390 a sheet). Two skewers is still under 250 calories.
The sauce is where it moves. Hong Kong curry gravy at a street cart is oil, curry powder and a lot of starch to make it stick. A tablespoon is about 51 calories. Most skewers pick up two to three tablespoons of coating, so the sauce can be worth more than the fish balls. If the cart lets the skewer drain before handing it over, you are closer to 100. If it comes dripping, closer to 150.
The real issue is sodium, not calories. One skewer of six with curry gravy carries around 600mg of sodium, nearly a third of the 2,000mg daily limit. Add the sweet sauce and chili that most people squeeze on top and it goes higher. So while this is the lightest thing on the cart for calories, it is one of the saltiest, which matters if you have blood pressure to watch.