  Why are online community sizes so extremely unequal? Most answers to this
question have pointed to general mathematical processes drawn from physics like
cumulative advantage. These explanations provide little insight into specific
social dynamics or decisions that individuals make when joining and leaving
communities. In addition, explanations in terms of cumulative advantage do not
draw from the enormous body of social computing research that studies
individual behavior. Our work bridges this divide by testing whether two
influential social mechanisms used to explain community joining can also
explain the distribution of community sizes. Using agent-based simulations, we
evaluate how well individual-level processes of social exposure and decisions
based on individual expected benefits reproduce empirical community size data
from Reddit. Our simulations contribute to social computing theory by providing
evidence that both processes together---but neither alone---generate realistic
distributions of community sizes. Our results also illustrate the potential
value of agent-based simulation to online community researchers to both
evaluate and bridge individual and group-level theories.
