While it's compassionate to treat all people with respect and dignity, you can't do that whilst at the same time lying to people. The unfortunate truths in this case are that overweight people are generally speaking less attractive than people in shape, and that there are health risks. It's not fat-shaming to talk plainly about facts. However, it's also not necessary to remind overweight people about these things constantly, and we should recognise that there are multiple factors that cause obesity, so it's generally unhelpful to be judgemental about it.
These things being said, if people want to be proud about being fat, then I'm okay with that, just as I could be proud about being an alcoholic or a lazy person - it's just a bit weird. But, if people want me to respect them for those things, I can't really do that because I believe them to be bad, and my words and behaviour will inevitably always reflect that view.