Bigger & Bigger BMI. The categories overweight, obese, & morbidly obese are defined by the body mass index (bmi), which is a measurement that looks at one's weight in comparison to his height. Tall people are allowed to weigh more than short people are. Bmi calculators are available at the doctor's office and also online. An adult with a BMI of 25-29.9 is overweight, 30-39.9 is obese, and 40 or more is morbidly obese.