That a good question. Let's see what the definitive sources say:

• medterms.com: A person has traditionally been considered to be obese if they are more than 20 percent over their ideal weight. That ideal weight must take into account the person's height, age, sex, and build.

  Obesity has been more precisely defined by the National Institutes of Health (the NIH) as a BMI of 30 and above. (A BMI of 30 is about 30 pounds overweight.)

• cdc.com: An adult who has a BMI between 25 and 29.9 is considered overweight and an adult who has a BMI of 30 or higher is considered obese.
• wikipedia.org: Obesity is a medical condition in which excess body fat has accumulated to the extent that it may have an adverse effect on health, leading to reduced life expectancy. Body mass index (BMI), which compares weight and height, is used to define a person as overweight (pre-obese) when their BMI is between 25 kg/m2 and 30 kg/m2 and obese when it is greater than 30 kg/m2

  Above we have multiple definitions for obesity. One thing stands out and that is the diagnosis is proportional to one's ideal weight. For many people it is not hard to put on an additional 20 to 30 pounds. You can easily put on 10 pounds at Thanksgiving. Fighting obesity is a tough battle for sure!