You want to write a line in a large file, use this procedure to do this.

procedure AddLine_InFile(sLine: String; const FilePath: string);
{
sLine = String that go to the end of file.
FilePath = FullPath of the file.
}
var
pLog: Textfile;
begin
try
  AssignFile(pLog, FilePath); // Assing a Path to TextFile.
  if not FileExists(FilePath) then // Check Existence.
   Rewrite(pLog, FilePath); // If not Make a new file.

  Append(pLog); // Set Current line as the end.
  WriteLn(pLog, sLine); // Write sLine if file.
finally
  CloseFile(pLog); // Close the file.
end;
end;