It would be interesting to correlate that with birth rates based on socioeconomic status in order to disentangle any confounding variables which might be present. For instance, more affluent people are generally better educated; better educated people are, according to the studies I've seen, less inclined towards religion. It stands to reason, then, that you'd have a preponderance of religious people in the lower classes; while their birth rate might be higher, it would be difficult to ascribe that to any single cause, since religiosity, socioeconomic status, or (the relative lack of) educational attainment might be to blame. They'd have to control for all of these factors in order to reach a more firm and meaningful conclusion.
I don't doubt the premise, however -- that being that religiosity leads to more reproduction -- if only for the fact that "be fruitful and multiply" is a pretty well-accepted religious mandate (other factors, such as the relative rates of contraception usage, can also be viewed as educational issues within communities, though these too are difficult to extricate from one another, and indeed are themselves frequently related to religious doctrine).
