I'll just link to
this study as "food" for thought. While obesity is correlated with health problems, it's not a foregone conclusion that simply having a certain BMI entails poor health. Often people who are obese also have unhealthy habits, for various reasons (obesity is linked to societal factors like poverty and lack of access to healthy options). When you track habit-formation over time, the difference between various BMIs is much less significant.
I'm not an epidemiologist, so I won't pretend to know whether the statistical analysis in the article is correct (it was peer-reviewed, so it at least seems plausible that it is). However, it's worth thinking about when making the jump from "that person is fat" to "that person is unhealthy."
From the article: "Healthy lifestyle habits are associated with a significant decrease in mortality regardless of baseline body mass index."