The size of a planet affects its gravity, its climate, and its likelihood of developing life.
Earth is large enough for an advanced civilization to develop. A smaller body such as Europa, while able support life as well, might not be large enough to birth an advanced civilization.
It is unreasonable for smaller bodies such as asteroids, to ever develop life? Also, what is the smallest a planet can be before it is no longer feasible for an advanced civilization to develop on it?