The medical definition of obesity is that People are considered obese when their body mass index (BMI), a measurement obtained by dividing a person’s weight by the square of the person’s height, exceeds 30 kg/m^2 (Reference: http://en.wikipedia.org/wiki/Obesity).
Now, this figure of 30 has been established by the medical community through statical analysis of a large sample for a large time. This is my assumption. However, this assumption can be considered safely as this is the general practice for creating any reference for in most forms of sciences.
However, taking measurement for any person and categorising him/her as obese may not be a right approach. It is true that obese people tend to have a number of diseases like diabetes, high blood pressure, etc. Such diseases can lead to more diseases like blindness or kidney disorder, etc. It is also true that a lot of obese people who are extremely fit and are carrying out a lot of useful work in the society and are not suffering from diseases to a level far different from people who are not obese.
Obesity cannot be considered as caused by eating too much. While this argument could be used a political statement to create panic for future food shortage, it is not scientifically valid. While obesity can be caused by eating too much, obesity can also be caused due to eating less than required and/or eating irregularly and/or living in extreme state of stress. Irregular food habits or stress can cause the body producing substances like collagen, etc. which settle around the abdomen area and deposit as fat. This leads to obesity. As with natural laws, excess of any substance can lead to disorder and these disorders can lead to diseases.
We cannot deny and wish away the changing demands of life on this Earth. As population is increasing, demands for resources are increasing. This is leading to more competition and thus leading to stress. Under these stressed situations, time available to people for essentials like exercising and/or spending relaxed time with family is reducing. All of these factors are leading to obesity. Thus, pinning down obesity on excessive food consumption is incorrect. What is despicable is that such a hypothesis for causing panic for future stress on food availability is being propounded.
With changing demands of life, stress cannot be wished away. So, instead, it makes sense that we create technology to reduce work load on human, so that they have more time for relaxation and more time for themselves (for essentials like exercises). This can lead to healthier life style and thus lead to reduction of obesity.