In Ayurveda underweight and overweight are signs that the vata, pitta, and kapha systems are out of balance. This can happen irrespective of how much or how little food you eat. Weight control problems happen when your food intake is not balanced by a proper digestion or metabolism.

Overweight or obesity is mostly found in people with predominantly kapha-type constitutions. This dosha is composed of water and earth. When it is imbalanced, the heavy characteristics of these elements become exaggerated.

However, obesity can be found in people with the other two doshas also. For example, people with pitta dosha are generally of medium build. But they can suffer from poor digestion if their digestive fire is too low because they have burned themselves out. As a result, food isn't processed properly, and that, in turn, can result in weight gain or obesity. 
For people with vata dosha the problem generally is that of underweight rather than overweight. They cannot properly absorb nutrients. However, they can become overweight when imbalanced if they attempt to eat sweet, oily foods in an attempt to calm their vata imbalance.

The Ayurvedic approach to achieving your ideal weight is based on taking a realistic look at your body type and what it can be, and then nourishing it back to natural health. It is about getting to know your body and becoming in tune with its rhythms.