Tomyam is genuinely one of the better options at a hawker stall if you are watching calories. A standard bowl of campur, clear broth, 400g, comes in around 250 calories with 22g of protein from the mixed prawns, squid, fish and chicken. The broth is mostly water, lemongrass, galangal, chili and fish sauce, so it adds very little. The calories are mostly from the protein and the chili paste base cooked with oil. That said, these numbers are ballpark estimates because hard data for Malaysian hawker food is thin. Different stalls use different amounts of oil, different ratios of paste, different amounts of protein. Treat 250 as a reasonable middle estimate, not a lab-verified figure.
What actually changes the maths here is a) how heavy-handed the cook is with cooking oil and b) whether you ordered putih or clear. The clear broth version stays under 260 calories pretty comfortably. Order tomyam putih (the creamy one with santan) and you are adding roughly 90 calories from 50ml of coconut milk, pushing the bowl to about 340 calories. That is 36% more calories from one ingredient swap, bro. The ayam version (chicken only) actually comes in lowest at around 200 cal. Chicken breast and most seafood like prawns and squid are roughly similar in leanness, but campur bowls tend to include fattier cuts like fish fillet and vary more in how much oil goes in, which pushes the calorie count up slightly. If you are eating this as a main and want to keep calories low, go clear broth, ask for extra protein, skip the rice on the side.