While the food is in the large intestines the body is reabsorbing some of the fluid before it passes as waste. If you evacuate your bowels too soon you run the risk of dehydration. The calories from the food you eat are actually absorbed in the small intestines and laxatives don’t affect the small intestines. This means that while the intent behind taking laxatives is to decrease the amount of calories absorbed by the food, the only thing these medications do is cause the large intestines to evacuate AFTER the small intestines has extracted all of the necessary calories.
After losing fluid from an early bowel movement the body compensates by retaining fluid. So laxatives cause an initial dehydration which makes the person ‘feel’ thin, but within hours the body compensates by retaining the remaining fluid and making the person ‘feel’ bigger.