why do they keep talking about 56 kg like its obese..?
One thing is that Japanese women are very short, which skews the numbers, in some cases quite a bit (there are all these 42 - 46 kg short VTuber girls).
Another thing is that Japanese girls and women are a bit (too) obsessed with being thin. I mean, it's healthier than being obese, but at the same time, it can be easily too extreme in the other direction - and there indeed are some reports about it being a bit more serious than "just a trend" over the years.