Is it true that thin people that have a large belly could be deficient in protein?

Yes. The big bellies of the you see in poverty stricken 3rd world is a dramatic protein deficiency. The calorie starved body breaks down muscle mass when their are no proteins in the food. This leads to the thin/wasted appearance. As the bodies protein level continues to decline, body water that would remain in the blood seeps into the internal organs like liver, spleen or the walls of the intestines.
Yes. Proteins in your serum absorb water into the blood stream vs. Letting it leak into the abdomenal cavity. Look up kwashiorkor.