Larger people have larger metabolic loads. Since water is the key to fat metabolism, the larger person needs more water to metabolize fat more effectively. Water also helps to maintain proper muscle tone by giving muscles their natural ability to contract and prevent dehydration of muscle tissue. Water also helps to prevent the skin sagging that usually follows weight loss. These shrinking cells become buoyed by the water and plumps the skin, which give a clear, healthy resiliency.
Water helps rid the body of waste and can relieve constipation.
During weight loss, the body has an increased amount of waste to get rid of. Adequate water helps to flush out the metabolized fat and other excess waste. Also, if the body gets too little water, the body will siphon what it needs from internal sources, like the colon, and constipation can happen. Water is an important part of waste elimination. This amount will vary from person to person.