Transiting planets have had their surface-area measured directly. On assumption that they're not spinning out like Saturn: therefore radius and a good constraint on volume. Mass too if they wobble their stars at all - giving us density.
Some are "super Earths" in the 1.3 Earth radius range... but others are quite a bit larger, 2.4 radius, and aren't wobbling the star enough to be iron or even rock. The only planets we have around here like that are Neptune: ice or even gas giants, covered in helium and hydrogen.
This is hard to square with their presence so close to a hot and possibly flaring star. Should they not be rocky? (Lava counts. Liquid metal-silica covers about the same volume as plain rock, as Archimedes noted, and weighs the same of course.)
The French have another idea: supercritical water. At a certain pressure / heat, the water is in supercritical mode - like carbondioxide on Venus' lowlands. It turns out that this extends the opaque part of the visible surface area. Like Sudarsky's class-II Water Giants, but smaller and hotter.
UPDATE 8/26/21: Hycea.
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