Omurice is fried rice wearing a disguise. The rice inside is cooked in butter with ketchup, and ketchup is a quarter sugar by weight. A 220g portion of chicken rice has 10g of butter and 25g of ketchup in it, which is 100 calories of fat and sugar hidden inside what looks like plain orange rice. The omelette on top is 2 eggs at 150 plus another knob of butter to get it glossy. Then more ketchup on top.
The fancier the sauce, the higher the number. Ketchup is about 20 calories for the squiggle on top. Demi-glace sauce, the dark one at yoshoku restaurants, is 100 to 150 for a ladle because it is stock reduced with butter and flour. A cheese omurice with demi-glace and a cream sauce is 900 to 950 calories. Pomunoki's large size is about 500g of rice and clears 1,200.
There is not much protein for the size. A 750 calorie plate has about 22g of protein, most of it from the 2 eggs, because the chicken in chicken rice is 30g of diced thigh at most. Compare that with a 468 calorie plate of 5 karaage carrying 28g. Omurice is comfort food with a big carb load, and it is best treated as the whole meal rather than a main with sides.