The oysters are almost irrelevant to the number. 60g of small oysters is about 45 calories and 6g of protein. What makes orh luak 487 is the cooking. Two and a half eggs at 190 go into a wok with about 20g of oil, which is 180 on its own, and the starch batter that makes the crispy, sticky texture is another 60. The chilli dip is 12. So about two thirds of the plate is egg and oil, and the oysters are a garnish with a name.
The HPB numbers show the size problem. The 2014 survey put a plate at 487. The 2015 one, with a bigger plate from a different stall, put it at 645. That is the same dish and a 158 calorie gap, because the stall used more egg, more batter and more oil to fill a bigger plate. A S$6 plate is the first number. A S$10 plate is the second. There is no version of this dish where the cooking is light.
The crispy style takes more oil than the gooey style. Crispy orh luak is fried longer in more oil until the batter is like a lacy cracker, and it lands nearer 600 for a standard plate. The wetter, softer style with more egg and less batter is closer to the 487. If you have a choice, the messy looking one is the lighter one.