It's not religious belief in itself that is helpful with health, but many religions are set up to promote social collectivism and keep the family unit together, which helps with health as it provides a good, stable environment to raise children in.

Also, just because a group has many children does not mean they are more healthy. There is alot of overweight, fat Christians in the US, for example. However I would say that as a group, Mormons are physically healthy because many of them focus on family helping eachother in hard times, providing a social net for fellow Mormons, and because they don't smoke, drink, or do drugs.