Question

How is the size of a planet related to the thickness of its atmosphere

Answer 1

Mainly, the mass of a planet doesn't matter, considering many planets are made up of gas, and how in different situations, atmospheres can be destroyed (The Sunburnt off Mercury's Atmosphere). However, in some cases, the planet can be in a good situation on the galactic map and have great conditions for an atmosphere, such as Earth. In which case, the mass of an object can attract a smaller object, causing gravity. Therefore, the bigger the mass of a planet when in right and specific conditions, the more atmosphere it can attract, if there is any floating by the planet.

If it is bigger it will have more gravitational influence and therefore gather more gases.