There are many reasons that cause the stomach to become bigger, and the more common reasons are as follows.1. Flatulence, all kinds of reasons lead to the abdominal gas is difficult to discharge or discharge too little, will cause the abdominal distension.2. All kinds of abdominal occupations, benign or malignant tumors, is easy to cause abdominal distension when it is relatively large.3. Long-term constipation, fecal matter accumulates in the intestinal tract, will also cause abdominal distension and pain.4. Ascites, such as ascites caused by liver cancer, liver cirrhosis or tuberculosis, etc. Ascites caused by cirrhosis of the liver or tuberculosis. In addition, if a woman is pregnant, abdominal distension can also occur, generally less common, but should also be taken seriously.