Delay in writing values to CSV using FasterCSV gem

Question

I'm reading values from one csv file using FasterCSV gem and doing some operations and dumping the same data to a different csv file by adding one additional column which specifies the status of the each row. Below is the code for the same

   path ="#{RAILS_ROOT}/data/reports/file1.csv"
   output_path = "#{RAILS_ROOT}/data/reports/file2.csv"
   FasterCSV.open(output_path, "w") do |csv|
   FasterCSV.foreach(path,:headers => false) do |row|
      if self.import_file_has_header == true
        if starting_row == 0
          row << "Status"
          csv << row
          starting_row = starting_row + 1
          next
         end
      end
     ActiveRecord::Base.transaction do
       # doing some operations 
      end
     row << "Success"
      csv << row
     end
   end

even the operations were successful in a transaction, there was a delay in dumping data ,this one observed when downloaded the file, which shows 0 MB at begining after some time size of the file is growing and for some time it will be of same size and the process will be continued. Is there any reason for this or am I doing anything wrong ?

I am using ruby 1.8.7 and fastercsv (1.5.4).