Obesity is that state of your body weight which is grossly disproportionate to your helght and build. BMI > 30. Body Mass Index is body wt in Kgs / (height in metres)^2.
India which is known to chronic malnutrition has obesity at 2 % and over weight also at around the same rates while US has obesity at 30% of population and overweight at 70% of population. (as given by the charts attached..) Have a look at the article also ..