What could be the underlying causes of significant abdominal distension or a large belly that is not attributed to excess fat?

This type of protruded stomach is caused by a damaged liver. The damaged liver is causing leakage of fluid and accumulation of a protein fluid in the abdominal cavity. 

With this condition, there is a replacement of the normal healthy liver cells with fibrous scar tissue, and you end up losing the function of the liver cells.  Cirrhosis doesn't happen overnight. It takes years to develop. This could happen from consuming alcohol, or from years of poor eating. It usually starts with inflammation, and typically the person will have a fatty liver as well. The inflammation is part of an immune response, and the body heals with scar tissue. 

Typically, what's done medically, is the lower the person's sodium, and put them on diuretics. 

Here are a few natural things you can do:

1. Lower your carbohydrates 

2. Stop consuming alcohol 

3. Consume moderate protein and fat 

4. Consume cruciferous vegetables

5. Intermittent fasting 

6. Give it time (3 years)