Long story short, I have an Earth-like planet, which is orbiting a [*mega*-Earth](https://en.wikipedia.org/wiki/Mega-Earth) (think a super-duper-sized version of a [*super*-Earth](https://en.wikipedia.org/wiki/Super-Earth)), which is orbiting a [K-type star](https://en.wikipedia.org/wiki/K-type_main-sequence_star), which is orbiting [UY Scuti](https://en.wikipedia.org/wiki/UY_Scuti).

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](https://en.wikipedia.org/wiki/Standard_gravity) - so I decreased the density instead.

As of currently, the relevant parameters for the mega-Earth are **approximately** as follows:

* 24.5 [M](https://en.wikipedia.org/wiki/Earth_mass) (note that this is approximately 25.75% of Saturn's mass, and at least 150% of Neptune or Uranus's mass)
* 3.5 [R](https://en.wikipedia.org/wiki/Earth_radius) (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](https://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html) 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](https://en.wikipedia.org/wiki/Hypergiant#Formation), 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](https://en.wikipedia.org/wiki/Mini-Neptune); I want a planet which has an oxygen-nitrogen atmosphere, rather than a hydrogen-based, [volatile](https://en.wikipedia.org/wiki/Volatiles)-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](https://en.wikipedia.org/wiki/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.