CrossFit workouts can increase or decrease your weight by either helping you shed fat or build muscle. Whether or not you gain or lose weight depends on your eating habits. If you consume fewer calories than you are burning during your CrossFit workouts, you will lose weight. If you consume more calories than you are burning, you are likely to gain muscle mass. Gaining weight while doing CrossFit could be a very good sign that you are increasing your lean mass and improving your body composition.