Planetary Atmospheres Venus ($0.815$ Earth mass) supports a $\rm{CO_2}$ atmosphere of $93 \ \mathrm{bars}$ at its surface. Earth's $\rm{N_2/O_2}$ atmosphere has a surface pressure of one bar.
What are the factors that limit the mass of atmospheric gases on a planet?
 A: One would  start by the molecular composition of what constituted finally a planet. 1 bar is approximately 1 kilogram per square centimeter. The heavier the gases found on a planet the larger the pressure on the surface given the same size.
Now the reason why Venus has heavier and/or more gases and earth less so, is another story.
Pick in the planetary lottery :).Planets do not have the same molecular composition.
Extra heat from the sun with respect to earth and closer approach should play a role in light gases escaping from the atmosphere. In the wikipedia article on Venus there are more  explanations offered.
A: Large planets tend to be gas giants, where Jupiter is gas all the way to a core made of metallic hydrogen and some rock.  The pressures inside there are very extreme.  A gas giant up to some 50 times the mass of Jupiter is a brown dwarf, and more massive than that and you start getting fusion.
A: It mainly has to do with the planet's distribution of volatile (gaseous at ambient temperature) substances, and the distribution of them.
On the Earth, most of the $\rm{CO_2}$ is bound up in carbonate minerals. High temperatures favor silicates over carbonates, and drive off the $\rm{CO_2}$ as gas. The reverse reaction, $\rm{CO_2}$ plus silicates is said to require liquid water (I don't know the reason), and hence isn't available on dry Venus (I think the hydrogen was lost because Venus lacks a magnetic field to shield it from the solar wind), and the $\rm{CO_2}$ ends up in the atmosphere. On Mars, $\rm{CO_2}$ is a major component of the atmosphere, but the low temperatures mean it condenses out as frost during the polar winter, so the atmosphere can't contain very much.
Also, the history of the planet can also have an effect on the amounts of volatiles. If the planet was very hot during some phase of its history the volatiles may have been lost at that time. The Earth probably lost most of its volatiles during the massive Moon-forming collision, and the current inventory of volatiles probably came from asteroid and comet impacts since that time.
