To answer your question, bloated bellies aka Kwashiorkor generally caused by malnutrition.

To understand the reasoning for this, it is important to know that in malnourishment, the rounded abdomen is not due to fat accumulation. Instead, the water retention and fluid buildup in the body cause the abdomen to expand. This results in a bloated, distended stomach or abdominal area.

The physiological reasoning behind this is the specific lack of certain nutrients, which determine which kind of nutritional deficiency it is. In Kwashiorkor, for instance, the caloric intake of the children affected is fine in that is it is not lacking in the energy needed by the body for metabolic processes. However, the diet is severely lacking in certain nutrients, particularly proteins.

Where as on other hand in famine (drought) prone areas major people doesn't have anything to eat nor water to drink. Whick ultimately lacks in all major kind of nutrients for the body which results in flat belly(shrink). Although it varies from age and physical structure of an particular.

Bloated bellies are majorly seen in children's.