We are the fast-growing religious group in the world because we bring peace and everybody knows the truth that Islam is the one and only religion God sent to mankind.
Muslim population is expected to grow at a faster rate than the non-Muslim population due primarily to the young age and high fertility rate of Muslims. Religious conversion has little impact on Muslim population, since the number of people who convert to or leave Islam are roughly equal. (Pew Research Center).
Muslim men are allowed to have more than one wife and correspondingly they must have lots of kid. In the other hand, Muslim women are not allowed to have more than one husband because of "one vagina one penis" rule is noble rather than "one vagina four penises" rule. Indeed, woman are forbidden to experience pleasure of another man while their husband aren't.
Muslim is taught to have good quantity of Islam followers and it is not necessary to be followed by the quality being a man. Quality of brain increases Muslim man to convert as an atheist faster and they are hard to convince to kill theirselves for the good of God as well. So, keep them low are the best result to bring glory of God on Earth.