First, another player quotes the Warcraft Realms stats page, and says that Zulu actually has less players on it then Azuremyst, a realm that was chosen as recommended. But Drysc answers that by saying Blizzard doesn't take "total players created" on a realm as a sign of population-- they're looking at percentage full (actually logged in) on any average night. Azuremyst is 35% full on average peak, and Zulu is 45% full on peak. He says once they fill those lower pop realms up a little more, they'll change the recommendations, and try to keep player populations as even as possible.
He also says (and he wouldn't lie, right?) that player numbers are actually seeing a "steady increase" since the expansion, but with the free transfers before the expansion and all the new landmass, servers might seem a little emptier. If you're cynical enough to think Drysc would twist the truth about player population, then you basically just heard him say "up is down."
But the strategist in me sees a mischievous idea here-- if Blizzard is watching actual logins during peak and not just created characters, reroll locusts could affect the recommended realms list. If you somehow got a big group to jump off of Zuluhed during peak, and on to Azuremyst during peak for a week or so, would Blizzard switch the two servers around? Could you actually game the realm population system?