I think you're missing the point of the body positivity movement. It's not celebrating being fat. It's celebrating being different shapes and still being healthy because some of us, women especially, have really poor body images, and we will never be stick figures, hard as we might try. It's to avoid young women getting eating disorders or thinking they must look like Barbie dolls to be acceptable.
Also, there's been studies done that show doctors have bias against overweight patients
Doctors fail when they conflate weight and health, believe their patients to be lazy or noncompliant or avoid discussing the topic altogether.
www.nbcnews.com
And lastly, weight alone isn't a good measure of overall health. Skinny people can be horrible eaters with high cholesterol, and there can be overweight people who are healthy, or perhaps someone is overweight but has an underlying condition causing weight gain (I personally think disorders causing weight gain are grossly underdiagnosed).