If you have a constant problem with water retention, excess salt may be to blame. Your body will tolerate sodium only in a certain concentration. The more salt you eat, the more water your system retains to dilute it. As it's forced through the kidney, it takes away excess sodium. The overweight person needs more water than the thin one.
Larger people have larger metabolic loads.
Since we know that water is the key to fat metabolism, it follows that the overweight person needs more water. Water helps to maintain proper muscle tone by giving muscles their natural ability to contract and by preventing dehydration. Water helps rid the body of waste. Again adequate water helps flush out the waste. Water can even help constipation.