Are gas giants really an advantage?

[Lisif Deoral]

Has anyone a clue about how the game generates planets?
For example, it might:

• First determine size, then atmosphere, then composition (ice, rock or gas): in this case small and tiny planets can "get" only rock or ice compositions, or
• First determine composition, then atmosphere, then size: in this case we have a minor number of planets without atmosphere since gas giants must have one, or
• Determine all stats of a planet according to a single "table" (e.g., 1% of all planets are huge methane gas, 0.5% are huge methane ice, 1% are huge methane rock, 1% are large methane gas, etc.)

Can the planet atmosphere and composition types be modded, by the way? (I seem to remember that the size classes may be altered...)