It works so many muscles that it quickly finds the weak links. If your waist is increasing, my thinking is that your core/hips are underdeveloped relative to the surrounding muscle groups (I can't speak for aesthetics or how you look to others). You waist is what links the lower to upper body and it's essential to being able to recruit and apply the full force from the muscles in either area. This is why so many people are posterior chain nuts because it ties the whole body together. Being weak in this area invites injury and prevents you from properly utilizing your strength.