@wilddeath
well, she is working with demons... but rather than being babysitter, she is more like ambassador, and office worker (documents). So it's not like she should be any stronger than normal office lady. Chiharu on the other hand carried gospel in hands for several years straight, she got used to it.
But i hear you, they never really confirmed anything regarding Chiharu "transformation", they just went "Oh, she still looks like a human, so all is good", so there might be plot twist about it. I just wanted to mention that 17 kg isn't that much, especially when you get used to it, at the very least it can be done by a "human", aka it doesn't certainly mean that she turned into demon.