I'm happy to agree to disagree on how much water intake suits any individual, but there are definitely different things going on in different sized bodies.A large person has a different surface area to volume ratio, and they have more difficulty shedding excess heat. So they are likely to perspire more - needing to replenish that with extra liquid.
They will also (if they have gained weight above their body's threshold), have grown extra fat cells. Normally we keep roughly the same number of fat cells through our lives, but if we get fat enough we grow more. These additional cells need additional fluid too.
And i would be far more concerned with damage to the kidneys from drinking too little, than from drinking too much, water. My (very basic) understanding of osmosis is that it is that to pull urea and other stuff out of a concentrated liquid (the blood) and through a membrane (the kidney), and then into the bladder, it is easier if the liquid in the kidney is less concentrated (because of drinking more/enough water). It is all to do with the concentrations. The more concentrated the urine, the harder on the kidneys because of the chemicals in the urine that need diluting.
But regardless of whether anyone agrees or not on how much water is the right amount, there are some very simple tests for dehydration (pinching the back of the hand, lifting skin just above the jawline on the cheek, colour of urine, etc.) i know from these, that my body needs the water it is getting, and is not happy with less.
Edited: for some really gaumless typos!
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But I suppose that small animals are more likely to become dehydrated faster, as they're not going to have such a large capacity "reservoir" as we (or large animals) have, and the same would probably apply to babies and very small children.
