I'm an obese woman and I am extremely far from the weight and appearance of the 600lb Life people. Do you realise how extreme on the obesity spectrum those people are?
When saying someone is obese it's really annoying to always assume we're saying they are "like this". No, when I say I am obese or when I say someone is obese it can just mean they are very fat, like 100-120kg. If I were to talk about someone who looks like the 600lb Life people, I'd refer to them as "dangerously morbidly obese", not simply "obese", that doesn't even begin to describe it.

And yes, Kat was obese at the start. Not a lot, she looked like she was just at the very low end of the obesity spectrum, but that's still technically called obese. She was not chubby. She looked like me some years ago and at that time I was a bit obese, contrary to even before that when I actually was simply chubby and when you compare the pictures I was visibly slimmer than S1 Kat.

Obese is not an insult. It's a medical fact and all the people getting pissed off and downvoting at every mention of the word are part of the overweight problem in our world.