Eating lettuce does not expand your stomach. Leafy green vegetables, such as lettuce and cabbage, contain a gas-producing starch called raffinose. Some people lack the enzymes necessary to break down this starch. As a result, their stomachs may feel stretched or bloated after eating lettuce.
Gas expands in the digestive system. As it does, it can cause discomfort until the body finds a way to either break it down or release it. Internal organs are not permanently stretched after eating gassy foods. Rather, the pressure from the gas causes a sensation that makes organs, such as the stomach, feel stretched.