The Water Giant thing is a naming I've never seen before either. I think the authors are mistaken since a gas giant is a gas giant is a gas giant. They just have different cloud-top temperatures at different orbit distances, unless they're so damn close that they boil off (need an orbital period of less than a coupla days before that happens!)

Of course, in the right HZ a gas giant would have water MOONS, as I described.

I own a copy of "Worlds without End" (Scrapped in near-mint condition by Marion County Library, Indianapolis and sourced second-hand from Amazon). It's a great book and also very simple - just the way I like planetary science.

The bit about large waterworlds and arid mountainais small worlds is entirely true. If you take the same composition but make Earth bigger or smaller, the amount of volatiles (atmsophere and ocean) scales as Radius^3 i.e. volume (or strictly, as mass, since gravity causes pressure which compresses larger worlds more). meanwhile, surface area scales as R^2. Therefore ocean depth and atmospheric "thickness" scale as R.

You can calculate that g approximately scales as R, too, although there is the compression density thing that complicates matters slightly. Therefore, atmospheric pressure scales as R^2. On a large waterworld, you'd have a thick soupy atmosphere above a deep ocean. [A gas giant is the extreme case of this] On a small world, like Mars, you'd have a thin atmopshere (not as thin as Mars - that is extreme because it's too cold and many gasses have mostly frozen out).

The Ocean thing - remember that mountains (and indeed continents) are held up by the strength of rock, which is fixed (or a fixed function of temperature, anyway) for fixed composition. On a world with stronger gravity, mountains would be lower. (Also, erosion wears down mountains and thicker atmospheres and deeper oceans on a larger globe with more coriolis force lead to more erosion and lower mountains - this partially offsets the tendancy of larger worlds to have more internal heat and more volcanoes)

Anyway, mountain height scales as 1/R or even faster.

Water doesn't end at the seabed - the mud and sand under the sea are wet, and the gaps between rock grains are wety all the way down through 5-10 km of crust, so the oceans we see are just the top of a deep column of water, mostly used up by rock.

Imagine a jar half-full of dry sand. If you pour about 1/10 jar of water in, the sand gets wet, but only up to about half-way There is still dry sand on top (unless you poured water on it when you filled the jar - so try putting 1/10 jar water in first, then adding dry sand). If you add 2/10 jar water, you get a shallow "ocean" 3/10 gives a deeper one, etc.

On a big world with higher g, the pressure causes the rock spaces to close up quicker as you go down through a rock pile (cementation and grain smearing are the main processes, but pressure is the forcing agent and pressure scales with g). So on a big world you first of all have more water, and secondly, you have less depth of porous rock to soak up the water, so you rapidly flood the continents (scaling with R^2, or thereabouts)

On a small world, the water soaks away into the sand and is gone. Nett result, very patch oceans, if any, and more land masses.

Juggling all those numbers, you can see that a world only 10% more or less massive than Earth would be quite different in surface features, even if everything else (plate tectonics, bulk composition, volatile %) were the same.

A world 10% larger than Earth would have 10% or less land, and with the more intense convection and volcanism, the land area would be relatively short-lived and keep being recycled.

A world 10% smaller than Earth would have around 50:50 ocean and land. Plate tectonics would probably be difficult or impossible on such a world. It might then build a few giant volcanoes, Tharsis style, or perhaps have intermittent catastrophic convection like Venus (only a few % smaller, but hotter and drier at the surface due to it's orbital position)

Inventing planets is fun, but you have to think quite hard about the consequences of turning the dials on the "oven"...