What you eat, how well you digest food, and disease are all possible to make this happen. Cause your stool has been large the last three weeks, not a short period of time, I guess you may have certain disease. Last year, I found my stools become very large one day. And the situation lasted at least five weeks! After visiting a doctor for diagnosis, I were told that I have celiac disease. Many other digestive disease may cause your stools to be large. So, if you didn't have something difficult to digest and the condition still exists in several days, you should go to see a doctor. Hope my answer will be helpful.