Enzymes. Even the thought of eating starts the release of digestive juices (enzymes). Stretching the stomach with food starts enzymes in the small bowel that cause fullness. This is usually a good thing and prevents over-eating. What amount is "eating so little? ".
May have. And ulcer in your stomach or swelling from inflammation or a tumor that limits gastric emptying and gives you early satiety, the feeling of fullness after very little food. Please see a gastroenterologis.