Birth control varies among Mennonite groups. It is not so much a matter of belief but more a matter of what size family one needs. The more modern Mennonites frequently live in urban settings so large families are not needed and birth control is used. On the other hand, many of the more conservative groups continue to live on farms or work in some farm related activity. They desire large families in order to have helpers on the farm so they would rarely use birth control.
Mennonites believe that children are of great importance. Jesus saw them as examples of how to receive the reign of God. Children are to be loved, disciplined, taught, and respected in the home and in the church. Children are also to honor their parents, obeying them in the Lord. Younger people are to respect their elders in the home and the church. (From Confession of Faith in a Mennonite Perspective)
Contraceptives are often recommended because of an understanding of the dangers of overpopulation on a crowded world. The concern about world resources is related to our understanding that we are to be good stewards of all that God has entrusted to us. This includes our concern for human beings as well as the world around us.
Every couple can make their own decision about family size. Some continue to have large families while others prefer smaller families. But the decision is not one that is mandated from church leaders.