how is the planets size like the size of is atmosphere
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Answer:
Howard Russel has it about right, but what most people are saying, that it’s the higher gravity, is mostly wrong. Planets which form earlier and grow larger tend to draw in a large envelope of hydrogen and other gas/ice materials, so if a planet is massive enough, its mass can contribute to its atmosphere—but this pretty much only applies to gas and ice giants with enormous oceans of mostly hydrogen and helium. Below a certain size threshold, planets will tend to be dry rocky dwarfs, like Mercury, Venus, Earth, or Mars. You might notice that the mass or size do not correlate very well with the atmospheric thickness in these four. Venus has by far the thickest atmosphere, but Earth is the largest. Mercury is a bit smaller than Mars and has no atmosphere while Mars has a bit of atmosphere, however their surface gravity is almost identical because Mercury, though smaller and less massive, is more dense.