Som tam on its own is one of the lightest plates in Thai food. 100g of green papaya is about 40 calories, and the lime, chili, garlic and fish sauce add nothing meaningful. The 150 calorie total comes from about a tablespoon of palm sugar (55 calories) and a small handful of roasted peanuts (45 to 60 calories). Ask for no peanuts and less sugar and you are eating an 80 calorie plate.
The problem was never the som tam. It is the table. A standard isaan order is som tam, a bag of sticky rice and gai yang or moo ping. The sticky rice bag is 260 calories, the grilled chicken thigh is 300, and the whole meal lands around 700. That is still a decent dinner, but people think of it as a salad and go back for a second bag of rice.
Variants matter. Tam pu pla ra (with fermented fish and salted crab, no peanuts, less sugar) is about 80 calories. Tam khai khem (with salted egg) is around 210. Tam thai at 150 is the middle. If you want the most food for the least calories, pu pla ra wins, as long as you can handle the smell.