I want this mega-Earth to be as massive as possible while still exhibiting a surface gravity less than or equal to twice that of Earth. This means increasing the radius of the mega-Earth; however, increasing the radius of the mega-Earth means that either the density decreases or the mass increases. I don't want the mass to increase - or, at least, I want to keep it within manageable limits, such that the gravity stays equal to or less than 2 G - so I decreased the density instead.
As of currently, the relevant parameters for the mega-Earth are approximately as follows:
24.5 M🜨 (note that this is approximately 25.75% of Saturn's mass, and at least 150% of Neptune or Uranus's mass)
3.5 R🜨 (you could fit three and a half Earths side-by-side inside this planet)
42.9 times the volume of Earth (still ~¾ or ~⅔ that of Neptune or Uranus, respectively)
Average density of 3 grams per cubic centimeter/3000 kilograms per cubic meter
My question: is this density realistic for a terrestrial planet of this radius and mass? The Moon is known to have a density of approximately 3.344 grams per cubic centimeter/3344 kilograms per cubic meter, but I don't know if it's possible for that density to scale up; I'm currently of the impression that a larger planet would "compact" its matter to a greater density, enough such that the radius would decrease, thereby increasing the surface gravity.
Please ignore the fact that this planetary system is orbiting UY Scuti - a star that may only exist for a few million years, therefore not existing long enough to form a planetary system. Moreover, please ignore the fact that a habitable moon orbiting a habitable mega-Earth orbiting a sun which is itself orbiting another sun is a rather unplausible situation. These are both part of the story.
Moreover, I don't want a mini-Neptune; I want a planet which has an oxygen-nitrogen atmosphere, rather than a hydrogen-based, volatile-based, or steam-based atmosphere. This means that this reduced density cannot be due to a massive but un-dense atmosphere; I'm asking whether a 3 gram/cubic centimeter density is possible for a the crust, mantle, and core of a massive terrestrial planet.
Note that this planet does not need to have a magnetic field, but it should have enough silicates for an Earth-like surface.