By Ian Musgrave
The "Standard Model" of planet formation is "hot accretion" wherein planets are built up from smaller planetessimals via relatively energetic impacts which partially or wholly melt the growing protoplanet (hence hot) [1,2].
Under this scenario, all terrestrial protoplanets lose all of their initial atmospheres during the accretion process. This is confirmed for Venus, Earth and Mars by the ratios of atmospheric Xenon, Neon and other noble gases to that of the Sun and the interplanetary medium (suggesting that up to 99.999% of the original solar-nebula derived atmosphere was lost during the accretion phase) [3-6].
The subsequent atmospheres of Venus, Mars and Earth are due to outgassing from the mantle, with some contribution from post accretion cometary impacts [3-6,8].
The difference between Venus and Earth is not due to the Moon forming impact at approx 4.5 Bya [5,7] but rather is due to the runaway greenhouse effect on Venus. Earth and Venus have similar CO2 levels overall, but Earth's CO2 is mostly locked up in rocks as carbonates and such like from weathering (and biological) processes that could not operate on Venus [3-6].
The important thing is that the composition of the atmospheres of terrestrial planets depends on the oxidation state of the mantle, the cometary contribution and the rate of UV-polymerized CH4 haze/ S02 haze removal in the early atmosphere [1,5,8] and the location of the planet in a "greenhouse friendly one".
Any terrestrial sized planet in the habitable zone would be expected to have an atmospheric density not too dissimilar to the current density of Earth's, irregardless of the presence or absence of a large moon, provided that the planet is not so close to its primary that a "runaway greenhouse" occurs.
1) Wetheril GW, (1996). The Formation and Habitability of Extrasolar planets Icarus, 249, 219-238 (and references therin).