The rice is the problem, not the chicken. Khao man means fat rice: jasmine rice cooked in rendered chicken fat and stock, which takes plain rice from 130 calories per 100g to around 170. On a 200g scoop that is a 340 calorie base before any chicken lands on it. The poached chicken itself is honest, about 165 calories for 90g and most of that is protein.
The version you order matters more than the shop. Steamed chicken (khao man gai tom) is 596. Ask for gai tod (fried chicken) and the batter and frying oil push it to about 700. Pasom (half steamed, half fried) lands around 650. The tao jiao sauce is 45 calories for a small dish and the pickled ginger inside it is free, so do not skip the sauce, just do not drink it.
Compared with Malaysian or Singaporean chicken rice at around 607, the Thai plate is nearly identical. It is a full meal with 28g of protein. The trap is ordering piset (special) with extra chicken and extra rice, which is usually a 750 to 800 calorie plate, or adding a side of blood tofu and liver, which is more protein but another 80 calories.