Body mass index is often used as a way to set weight classes of underweight to obese, but the measurement is likely too basic. From the New York Times:
The illustrations here were created from scans of six people, who were all 5 feet 9 inches tall and 172 pounds. This means that though their bodies look very different, they all have exactly the same body mass index, or B.M.I. At 25.4, technically each of them could be considered overweight.
And Dwayne “The Rock” Johnson is classified as obese.
Of course, this applies to most indexes and classifications. You try to get it to work for as many as people, places, or things as you can, but there’s always going to be exceptions and more information to be had.