Question

why our country is most populated

Answer 1

Traditionally people were living in rural areas.  Many regions in India are fertile and well to do.  Long ago mortal rates of children were high.  People in India had the opinion that if they had a lot of children, at least some of them will grow to be adults and take care of them later on.  Parents felt that more number of sons , is better for taking care of their family.  Many parents had many children until they had sons.

Also, a lot of people migrated into India from other countries, essentially from the neighbouring countries.
 
In India divorces and the system of single fathers and mothers is less.  Undivided families and united families are more.   So there are more opportunities for the population growth.

Answer 2

our country is most populated because many parents think that they should have many children so that in future all of htem help tem in their old age. some parents only gets girl child so they born children until they had a son