Question

How does the size of a planet affected the amount and type of gas in its atmosphere

Answer 1

There are lots of variables that directly and indirectly contribute to the presence of gas on a surface
if the size of a planet is relatively small it will in turn be that of a smaller area which results in the less area to be covered for gas which basically means higher presence
I can go in depth more but I don't think that would be necessary all you need to know is this ...based on the size and gas will in turn be parallel to it's conformity