Udon gets treated as heavy because the noodles are thick, but a bowl of kake udon, just noodles and broth, is only 270 calories at Marugame's regular size. Kitsune udon is that bowl plus one piece of sweet fried tofu, and the tofu is where the 115 extra calories sit. Aburaage is deep-fried tofu, about 380 calories per 100g before it is simmered in sugar and soy, so a 30g pouch that has soaked up sweet broth comes to around 115.
The broth is close to free. About 300ml of dashi with soy and mirin is 30 calories, though it carries 3 to 4g of salt if you drink it all. Kansai broth is lighter in colour and slightly lower in salt than Kanto broth, but the calorie difference is nothing. Leaving half the soup saves 15 calories and half the salt, which is the better reason to do it.
The number climbs with the extras, not the bowl. Marugame and Hanamaru put tempura by the till, and one kakiage is 150 to 180 calories, a chikuwa tempura about 100, an inari sushi 100 to 120 each. A kitsune udon with a kakiage and 2 inari is 720 calories, nearly double the bowl. The large size (oomori) adds about 130 calories of noodles too.