Also, you're a bad person for saying a woman who doesn't shave her legs is gross.

That meaning is very different to saying "grossly obese" in the same sentence as never showering or shaving her legs. At worst Yvain could be bad for saying that people who are very, very, overweight is gross - and even then it wouldn't be somewhat of a distortion.

Writing simply 'obese' would be an underspecification. For example the only time I have ever qualified as officially 'obese' was when I was body building aggressively - which is an entirely different thing.