Asam rebus is basically fish boiled in a tangy tamarind broth with belacan, shallots, chili and a few aromatics. No oil, no thick base, no coconut milk. If you are not familiar with it, think of it as the cleaner, more sour cousin of asam pedas, the kind of thing your grandma makes when she wants something quick and not too heavy. At 180 calories for a full fish with broth, the numbers are genuinely good for a lauk, and that 23g of protein is doing real work. Fair warning though, these numbers are a ballpark estimate because detailed lab data for asam rebus is basically nonexistent. The figure is built from the fish flesh calories plus a very light broth, so treat it as a reasonable guide rather than a hard fact.
What actually changes the maths here is the fish size and whether anyone adds extra oil at the end, because some versions do get a small drizzle before serving. Swap ikan kembung for ikan merah and you are looking at roughly 190 calories, mainly because the fillet tends to be a bit bigger. Go with ayam kampung instead and that jumps to around 240 calories, though you get more protein from the larger meat portion, roughly 150g of chicken versus 100g of fish. The broth itself is basically free calories, around 20 calories for 200g, so drinking the kuah is fine. Where people quietly wreck this dish is by pouring it over a mountain of white rice without accounting for that extra 400 to 500 calories sitting underneath.