Sticky rice is not more fattening than white rice per dry grain. The difference is water. Steamed khao niao ends up at about 160 calories per 100g, while a scoop of steamed jasmine rice is about 130 per 100g. Same starch, less water, so every mouthful is denser. A small bag is 150g and 250 calories; the bigger bags some stalls sell are 200g and 320 calories.
The problem is what khao niao comes with. Nobody eats a bag of sticky rice alone. A bag with one piece of Hat Yai fried chicken is 570 calories. A bag with 3 moo ping skewers is 640. A bag with som tam and larb is a very reasonable 600 calorie meal with good protein. The rice is a constant 250; everything else decides whether lunch is 500 or 900.
One real advantage: sticky rice is slower to eat. You pinch off small balls by hand, and a 150g bag takes a while to get through, so most people stop at one bag. Compare that with a plate of rice under a curry, which disappears in 5 minutes. If you are watching intake, one bag of khao niao with a grilled protein and som tam is one of the easiest Thai meals to keep under 600.