This is certainly true in So Calif where I live. Just in terms of JWs, we've seen the population of our region double or even triple in some areas in just the past 15 years (and most of that growth occurred by 2005, when the recession began to slow things way down).
The number of "English" congregations is unchanged from the mid-1990s, and each one has about the same number of members. Which means their growth has been zero - or well below zero if you factor in the growth rate and look at what you'd expect given the surge in population.
The population growth here is largely (but not exclusively) due to immigrants from nearby Mexico and other South and Latin American countries. Not only have hundreds of thousands of immigrants moved here seeking jobs in recent decades but once they get here they send for their relatives, which spurs more growth. In addition, they tend to have large families.
Before we exited the borg, we shared a hall with a "Spanish" congo. As an elder, I sometimes had to stay after the meeting for various reasons. It was interesting to emerge from the library to find your own congo gone and a new one coming in the door; you got to contrast the make-up of the two congos. Most of the Anglo dubs had small families. Like other white suburbanites in the late 20th century, two children is the cultural norm. In addition, many of the publishers were older, often retired, with no children left at home. On the other hand, the Latinos were mostly all younger families with four, five, sometimes six children. It is commonplace here to walk into WalMart and look around: About 80% of the customers are Hispanic and a typical family is four children, all within a year or so of each other in age. This is in marked contrast to 15 years ago, when the ethnic make up of WalMart shoppers here was substantially more diverse.
Immigration and cultural differences regarding family size appear to be driving the growth of JWs (and other evangelical churches) in this region.
