People assume crab fried rice is the light option because crab is lean, and the crab part is: 50g of crab meat is 45 calories and 10g of protein. But it is still fried rice. A plate uses about 200g of cooked rice at 260 calories, one egg at 75, and a tablespoon or more of oil at 135. The Department of Health lists pork fried rice with egg at 557 for 315g, and crab fried rice lands in the same place, about 560 for 300g.
The price tells you the crab weight. A 60 baht stall plate has maybe 30g of crab, often mixed with crab stick, and is closer to 500 calories with a bit less protein. A 250 baht restaurant plate can carry 100g of lump crab, which adds protein (20g from crab alone) but only about 45 extra calories. If you are going to eat fried rice, the expensive one is the better deal per calorie.
The side plate matters too. Khao pad poo comes with cucumber, spring onion and a lime wedge, which is free, and a small bowl of prik nam pla (fish sauce with chili), which is about 5 calories. What is not free is the fried egg some places put on top for 10 baht: another 100 calories. Order the plate plain and the lime and prik nam pla do the work.