If you drop a capped collection and re-create it with different size params then data files never get cleaned up.

Dropping capped collections

require 'pp'
@conn = Mongo::Connection.new

def randomString (string_length)
  chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXTZabcdefghiklmnopqrstuvwxyz";
  randomstring = '';
  0.upto(string_length) do
    rnum = (rand * chars.size).floor
    randomstring += chars.slice(rnum,rnum+1);
  end
  return randomstring;
end

@size = 10000000
0.upto(100) do
  @conn.db('capped_test').create_collection('capped', {:capped => true, :size => @size, :max => 10000})

  #  Report storageSize and fileSize
  pp "storageSize: #{@conn.db('capped_test').collection('capped').stats['storageSize']}"
  pp "fileSize: #{@conn.db('capped_test').stats['fileSize']}"
  pp "-----------------------------------"

  #  Save the current storageSize for next iteration through the loop
  @size = @conn.db('capped_test').collection('capped').stats['storageSize']

  #  Drop our collection
  @conn.db('capped_test').collection('capped').drop()
end