The explanation for this is that one of the important ways of seeing a church is in terms of the people having radial relationships with the minister. Everyone wants to be known and to know the minister to some extent at least. With more than about 100 people in a congregation this becomes less easy.
The main solution to this is to have lots of small groups where everyone can know and be known both by the leader and by others in the group. Congregations that grow very large also have a variety of groups and programs that allow people to have direct relationships with leaders and others.
Another limitation that is well know concerns the size of the building. The rule of thumb is that when a building is 80% full it is practically full. Newcomers will feel that it is full and often have difficulty finding enough seats together for their family or friends. Although the building may overflow on some occasions the usual attendance will generally settle to around 80% of capacity.
Multiplying small groups helps congregations grow. Adding new congregations does too. At present the planting of new congregations seems to be the main method of allowing more people to participate in the life of the church. In the US the Assemblies of God planted a new church every day on average in 2011. Every new church they started in 2009 added about 50 new converts that year.
Dale