I can't see it, very few religions has so many members, and none of them younger than a thousand years.
Firstly: define 'religion'. Mormons would, of course, claim that they are Christians, not their own separate religion. Most Christians consider them to be, at best, heterodox. Still, for the purposes of this discussion, let's consider some branches of Christianity to be distinct.
Protestantism, if you want to lump all Protestants into a single group, has way more than 200 Million adherents - I don't know the numbers, precisely, but most people in the US count, plus most of northern Europe, plus...
If you want to look at individual denominations, Anglicanism by itself has a significant chunk of those numbers.
http://www2.anglican.ca/search/faq/021.htm said:
The Anglican Communion consists of "over 70 million members" organized in "38 self-governing Churches made up of about 500 dioceses, 30,000 parishes and 64,000 individual congregations in a total of 164 countries. While the Anglican Communion does not rank among the biggest groupings of Christians, it is, after the Roman Catholic Church, arguably the most widespread". [1]
---
[1] 2006 Church of England Year Book. London: Church House Publishing, 2005. "The Anglican Communion", p. 337.
Now, the Anglican communion grew on the foundations of the British Empire, and while much of its growth is happening today in Africa, it's still happening largely in former British colonies.
Mormonism is currently growing by leaps and bounds
http://www.lds.org/ldsorg/v/index.jsp?vgnextoid=84010fd41d93b010VgnVCM1000004d82620aRCRD&hideNav=1&query=church+growth+rate&bucket=AllChurchContent&submitSearch=Search said:
News of the Church
There are currently 8,254 meetinghouses internationally, which shows a 10 percent growth rate over the past five years. That trend is also reflected in the United States, where there are 6,361 meetinghouses—or a 9.6 percent growth rate for the same time period.
Gospel Library > Magazines > Ensign > June 2007