system
Given the following post, respond as if you're commenting on Reddit:
human
If Venus and Earth are approximately the same size, why is Venus' atmosphere so much thicker?

I'd heard years ago from a forgotten source that the reason is because of the fact that our moons mass is particularly high compared to ours (highest ratio in the solar system) and that over time the moons gravity had pulled a lot of our atmosphere out into space where it dispersed. 

When I mentioned this in a discussion about exoplanets and how it is likely a lot of them will be uninhabitable since most of them probably won't have moons as large as ours and therefore will have much thicker atmospheres everyone pretty much told me I was wrong and downvoted me.  I went to find the source for this but could not.  I left it at that, but then I remembered Venus, and how it has no moon and has a much thicker atmospere.  So I'm wondering, if the lack of moons around Venus as opposed to the very large moon around Earth isn't the reason for the much different atmospheres on these two planets, what is?
gpt
Most of Earth's CO₂ is bound in the carbonates (such as limestone). When CO₂ dissolves in water, it forms CO₃²⁻ anions, which react with several metal cations to form insoluble carbonate minerals, such as limestone or dolomite.

When you heat carbonate minerals, the CO₂ is released, and carbonates turn into oxides (for example, limestone CaCO₃ turns into lime CaO, releasing the CO₂ in air).

Since Venus has no surface water, but is geologically active (though not as much as Earth), it cannot reabsorb CO₂ into the crust as limestone, while it can release it into the atmosphere from mineral deposits. Earth has massive amounts of CO₂ bound in limestone, while pretty much all of Venus's limestone is in its atmosphere.

Regarding the atmospheric composition; the oxygen in Earth's atmosphere is of biological origin. The primordial atmosphere of Earth had no or very little oxygen.