Why would a cactus shrink in the middle?

When water is available, cacti absorb more than they need for the present use. Their tissues, particularly in the thick body or trunk of the cacti, and in the thick pads of the Prickly Pear, swell up like sponges. When the weather dries and no water is available through their roots, cacti use this stored water. Their tissues then shrink up as the moisture is used, and the stem and/or pads become noticably smaller and wrinkled.