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 kidneys. It takes away excess sodium. The overweight person needs more water than the thin one.
Larger people have larger metabolic loads.
Since we know the water is the key to fat metabolism, it follows that the overweight person needs more water. Water also helps prevent sagging skin that usually follows weight loss, shrinking cells are buoyed by water which plumps the skin and leaves it clear, healthy and resilient.