Barring surgery, an adult's stomach typically remains the same size regardless of food intake, according to WebMD. A person's stomach expands to absorb a large meal, reports the Academy of Nutrition and Dietetics, but it goes back to its normal size after passing food to the small intestine.
What people perceive as a reduced stomach size is really just retraining eating habits so that a person feels fuller after eating less, reports WebMD. Weight doesn't influence stomach size, as some thin people have stomachs equal to or larger than those of others who weigh more.
Some people have stomach-reducing surgeries to reduce their stomachs to the size of a walnut. This type of surgery, called bariatric surgery, is employed when an obese person is unable to reach or maintain a healthy weight through diet and exercise.