Tom saap is the lightest soup on an Isan menu that still counts as a meal. The broth is water, lemongrass, galangal, lime leaf, lime juice, fish sauce and chili, which comes to maybe 15 calories per bowl. Compare tom kha at 320 a bowl, where 180 calories are coconut milk, or tom yum nam khon with its chili paste and evaporated milk. Everything in a tom saap bowl that has calories is the meat.
The meat is pork ribs or kraduk on (soft cartilage bones). A stall bowl carries around 180g of bone in ribs, which gives roughly 90g of edible meat and fat at about 230 calories. Ribs are a fatty cut, so the fat number for the bowl is 18g even though nothing was fried. Pick tom saap moo sam chan (belly) and the bowl climbs to 380; pick the chicken version and it falls to about 200.
The catch is the rice. Tom saap comes with a bag of khao niao or a plate of rice, and that is where a 270 calorie bowl becomes a 520 calorie lunch. Still fine. The bowl also comes with toasted rice powder and chili flakes stirred in at the table, about 20 calories combined, so the numbers above already include them.