Answer:
There are two primary factors: size and distance from the Sun. Gravity helps planets and moons to hold on to their atmospheres, so small planets/moons such as Mars and the Moon have thin atmospheres. The reason why the outer planets are larger is probably down to two major factors. The first is simply the amount of material available which may be accreted to a planet. Clearly, the further away from the Sun, the longer the orbit (twice as far means an orbit twice the physical length).
Explanation:
