Enlarged abdomen after rectal cancer surgery may be caused by abdominal distension and ascites. After rectal cancer surgery, the small abdomen becomes bigger may be caused by abdominal distension, because the digestive function will be decreased after rectal cancer surgery, and the peristalsis of intestines will be slowed down, so the symptoms of distention and pain and the small abdomen becomes bigger will occur, which can be treated by taking oral medicines such as omeprazole and cisapride under the instruction of doctor. Enlarged abdomen after rectal cancer surgery may be caused by ascites. As ascites occurs after spreading and metastasis of rectal cancer, symptoms such as abdominal distension, lack of appetite, oliguria, and enlarged abdomen causing mobility difficulties will appear. It can be treated by drugs such as furosemide and spironolactone. Therefore, when the small abdomen becomes bigger after rectal cancer surgery, you should pay attention to light diet, eat more food rich in dietary fiber and nutrition, report to the doctor in time if there is any discomfort and actively cooperate with the treatment.