One takoyaki is about 50 calories and the octopus inside is 4 of them. The ball is batter, roughly 6g of flour with dashi and egg, cooked in a pan that is oiled between every batch. Some of that oil ends up in the crust. A tray of 8 has about 12g of oil from the pan, which is 108 calories, more than a quarter of the tray. That is before anyone reaches for the sauce.
The topping is a set routine and it costs about 70 calories. Sauce is 25, mayo is 40, and the aonori and bonito flakes are almost nothing. Where trays differ is the cooking style. A festival stall making soft, gooey takoyaki uses less oil and a tray is 350 to 400 calories. Gindaco fries the shell crisp in a lot of oil and its 8 piece tray is around 550 calories for 650 yen. Same food, 150 calorie gap.
Portion size is the real variable. Takoyaki is sold as a snack, but 8 balls is 200g of food and 400 calories, which is a light meal. A 12 piece tray at 600 calories is a full one. Since it is usually eaten standing, between other things, on the way to dinner, it is easy to forget that it counted.