Problem with downloading multiple files using AsyncTask

I'm using the following script based on the tutorial Android Series: Download files with Progress Dialog to download multiple video files from the internet to the SD card. It displays a progress bar while the download is in progress.

public class MyDownload extends Activity {

public static final int DIALOG_DOWNLOAD_PROGRESS = 0;
private Button startBtn;
private ProgressDialog mProgressDialog;

private String videoPath = "http://my_site.com/test_videos/";    
private String[] fileNames = {"file1.mp4","file2.mp4"};
private TextView tv;    


/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    startBtn = (Button)findViewById(R.id.startBtn);
    startBtn.setOnClickListener(new OnClickListener(){
        public void onClick(View v) {
            startDownload();
        }
    });
}

private void startDownload() {      

    tv = (TextView) findViewById(R.id.TextView01);       

    if(checkExternalMedia()==true) {

           File file = null;                
           for(int i=0; i

}

This code works fine when the size of each video is less (around 300KB). But when the video size increases to 7-8 MB the application crashes.

However, if I try to download a single large file instead of

private String[] fileNames = {"file1.mp4","file2.mp4"};

this code works fine. Any thoughts ?