Why are we even talking about this?
Because you started it, and I find it more interesting than the rest of this thread.
Well there is an assumption that I stated that probably wouldn't hold true. I'm curious if anyone can spot it.
Density.
Correct. for a given composition, the density rises as mass is added (self compression). Exo-planet density is also quite variable. For example Kepler-20f is something like 14 times the mass of Earth while being slightly larger in radius, but Gliese 1214b is about 6.5 times the mass of Earth while have a radius about 2.6 times that of Earth.