It's Worth Checking
"By "demographic," I mean the Muslim world's high birth rate, which by mid-century will give tiny Yemen a higher population than vast empty Russia."
A few minutes of googling got me to a page of summary demographic data from the U.S. Census Bureau. Their population estimate for Yemen in 2050 is 71 million, which is indeed surprisingly high. Their estimate for Russia, however, is 109 million.
Population estimates that far ahead are inherently uncertain; they depend in part on reproductive decisions by people not yet born. But insofar as there can be an authoritative source for such an estimate, the Census Bureau comes a lot closer to qualifying than Mark Steyn. Googling some more, this time on references to Steyn's book, I have not yet found anyone else who bothered to check his numbers. Reviewers and bloggers, at least the ones I found, simply took them at face value.
After all, it wouldn't have been printed if it wasn't true, would it?