Glass noodles are mung bean starch, and once they soak up water they sit at about 85 calories per 100g. That is half of what egg noodles or sen yai give you. A stall plate uses roughly 120g of cooked noodles, which is only 100 calories, so the noodle itself is not the issue. The remaining 235 calories are the egg, the pork and the oil in the wok.
The oil is the part you cannot see. A tam sang cook pours 1 to 2 tablespoons into the wok for each plate, and glass noodles soak it up like a sponge. That is 70 to 140 calories that never show up on the menu. Published Thai figures for the dish run from 226 to 265 for a small plate up to 350 for a full stall serving, and the spread is basically how heavy the cook's hand is with the oil.
One thing to know: glass noodles are pure starch with almost no protein, so this plate only gives you about 14g of protein, mostly from the egg and the 40g of pork. If you are eating it as a meal, ask for extra egg (kai song) or order it with prawns. Each extra egg adds around 75 calories but doubles the protein.