[GROSS SIMPLIFICATION]There are two main infuences on the solidity of a planet-
1/distance from the central star
all the inner planets in our solar system are rocky, but if they had formed further away from the sun more water and gases would have remained on their surfaces and they would have been icy planets- the amount of ice increases in the outer system because it is cold.
Also the now icy planets would retain thicker atmospheres, becoming similar to small versions of Neptune or large versions of Titan.
Move an icy world with a thick atmosphere inwards toward the sun, and the hydrogen in the atmosphere would evaporate, the ice would melt, dissociate and sublime, so eventually you would be left with a large water-rock world like the Earth, or a smaller dry world like Mars.
All the planets really can be placed on a two-dimensional grid, with mass as one axis and distance from the central star as another axis, and pretty good estimates made of how much water and other volatiles are retained,
and how thick the atmosphere is, etcetera-
it is all to do with heat and gravity, basically, although the history of the planet is very important too.
What has not been seen in detail is a large planet close to the primary- they exist in large numbers, for instance at Upsilon Andromedae (images here)
Such a world, if very large, would retain a hot atmosphere, and possibly even be heated to fusion point - a little smaller and the atmosphere might be a lot smaller and even absent.
It would be possible to imagine a very hot, rocky world near to the central star- but it could not be too big, or there would be some sort of atmosphere, possibly far hotter than Venus’s.
Large rocky worlds are dense, and so have greater gravity, and so are more able to retain atmospheres.[/GROSS SIMPLIFICATION]
What I am trying to work out ATM is how big an Earth type planet can be- if the planet is slightly less dense, and slightly further away from the central star, it might be possible to have an Earth-type planet twice as big as our own- or more…if you don’t mind slightly high gravity…