It's a result of 1) liberal imigration policies (and I'm not using that word in a political sense) and 2) cultural differences. With traditional moselm marriges taking place at young ages, the women are able to conceive more children then in western society where marriges take place later in life (after puberty) and growth rates are more culturally inhibited.
An important question to ask is if the middle east is univerally revered as sacred Islamic land among moslems, why are so many emigrating out of there and into Europe?