Batagor is siomay with an extra step, and the step is a deep fryer. A steamed fish siomay is about 51 calories. Fry it and it is 58 and then it sits in oil, and the tofu pieces are worse because tofu soaks up oil like bread. FatSecret's 290 per 100g figure for batagor is more than double the 138 per 100g for steamed siomay. Same fish paste, same tofu, different number.
Then the sauce. Batagor peanut sauce is thinner than siomay sauce but there is more of it, and 40g is about 135 calories. Kecap manis and sambal on top, another 30. So the Rp 15,000 to 20,000 portion is about 600 calories with 36g fat. That is more than a plate of nasi goreng, and most people eat it at 4pm as a snack.
The protein is not bad at 20g because the filling is real fish, usually tenggiri or the cheaper variety. Batagor kuah, the soup version where the fried pieces go into a clear broth, is around 420 because there is no peanut sauce, and it is the better order if you want the crunch without the full 600.
The Bandung specialists, the Kingsley and Riri type, give bigger pieces and more sauce, and a portion there is closer to 700. Cart batagor near schools uses smaller pieces and is around 450, but people buy two.