Gelman et al's BDA3 has a fun exercise estimating heart-disease rates in one of the early chapters that demonstrates this issue with effect-sizes. BDA3 uses a simple frequentist model to determine heart-disease rates and shows that areas with small population sizes have heavily exaggerated heart-disease rates because of the small base population. Building a Bayesian model does not have the same issue as the prior population prevalence incorporates the small base population sizes.