The pink colour scares people who assume it is sugar syrup. It is nam yen ta fo, a sauce made from fermented red bean curd, garlic, vinegar and a little sugar, and a bowl uses about 20g of it for around 20 calories. The broth itself is a light pork or fish stock at maybe 15 calories per bowl. So the two things that make yen ta fo look unusual add up to less than 40 calories.
Where the calories sit is the noodles and the toppings. 120g of sen yai is about 130 calories. A standard bowl carries 60g of fish balls, fish tofu and squid at about 70 calories, which is lean protein. The fried wonton skins and fried tofu on top are the one fatty part: 20g of fried wonton is around 50 calories, and a shop that piles them on can add 100 more.
The version to watch is yen ta fo with extras. Add pork cracklings (kaeb moo), extra fried wonton and a couple of pieces of pork blood jelly and a 290 calorie bowl becomes a 400 calorie one. The dry version (yen ta fo haeng) tossed with garlic oil is another 60 to 80 on top. Order it nam (soup), standard toppings, and it is about as light as a Thai noodle lunch gets.