Khao tom is rice stretched with water, which is why it is light. A bowl holds about 60g of dry rice equivalent, roughly the same as a small plate, but it comes as 300g of soup that takes 15 minutes to eat. That 300g of rice soup is about 200 calories. Plain khao tom in a Thai table is 120 per cup, and a stall bowl is about two and a half cups.
The toppings are the variable. Minced pork (40g, about 70 calories) is the standard. Khao tom pla uses about 60g of fish and lands at 325 per cup in the hdmall table because it comes with a bigger dose of garlic oil and celery. The garlic oil is the sneaky part: a stall spoons 8 to 10g of it on top, and that is 70 to 90 calories if they are generous, 30 if they are not.
The other version of khao tom is the khao tom kui restaurant, where you get a bowl of plain rice soup (about 200 calories) and order side dishes: salted egg, stir fried morning glory, omelette, pork with garlic. That is where a 200 calorie bowl becomes a 900 calorie dinner, but each side is a choice. Two lean sides (salted egg at 90 and stir fried greens at 120) keep it under 450.