The name means 'grilled how you like it', which is exactly the problem. The base is fine. A 120g pile of cabbage is 28 calories and an egg is 80. The 50g of flour batter is 180. What pushes it past 500 is the 40g of pork belly at 160 calories and then the table work. Okonomi sauce is 35 calories per tablespoon and mayo is 70 per tablespoon, and most people do not stop at one of each.
Hiroshima style changes the maths. The same pancake gets a full serving of yakisoba noodles griddled in, another 200 calories, and often a fried egg on top for 90 more. A 'Hiroshima special' with noodles, pork and extra egg is 750 to 800 calories for around 1,200 yen, roughly the same as a large katsudon. It also weighs close to 500g, so at least you feel it.
The upside nobody mentions is the cabbage. A single okonomiyaki holds 100 to 150g of cabbage, which is more vegetable than most Japanese meals manage in a day. Made at home with a lighter hand, half the flour, thin pork loin instead of belly, sauce brushed on rather than poured, the same size pancake comes in at 350 to 400 calories and still tastes like the real thing.