When you are talking about "overweight" and "obese", you are talking about the BMI.
Unfortunately your idea of disparaging the "clinically overweight" is based on arbitrary BMI cutoffs that are poorly chosen. Recent studies have shown that overall health outcomes (ie, not just heart-disease numbers) are actually better for "overweight" people than those of "normal" weight. And BMI does not scale with height correctly, so it imposes a much stricter standard of what's "healthy" on someone who's 6'3" as opposed to someone who's 5'3". The ratings for what's "normal" also do not take into account age or sex, much less things like frame size.
BMI was devised because height and weight are numbers that have been collected in medical records and studies for decades, whereas the other factors are far less likely to have been recorded at all. So while BMI is often used as a tool for doing large-scale historical research or research across large population groups, that's simply because BMI is a number we can easily figure out, and not because it's actually a good indicator of an individual's health.