Here's a simple answer to the more-massive-yet-smaller question for "cold" objects:

In ordinary matter, electrons are held to nuclei by their electrostatic attraction, an effect that generates atomic structure. But electrons screen out nearby nuclei, meaning that this effect is non-cumulative for large objects.

But gravity is cumulative over an entire object, and a more massive object has greater gravitational force, enough to make it smaller if gravitational effects are stronger than electrostatic ones. In effect, the atomic structure gets crushed.

Growth, then shrinkage with increasing mass implies that there is a maximum-size mass, and that mass is approximately the mass of Jupiter. And this shrinkage also explains why solar-mass white dwarfs are approximately Earth-sized; white dwarfs are relatively cold.

I can use some simple algebra to get some approximate numbers if anyone desires.