AVMutableComposition reduces Size and Resolution of final video

Question

I created a video using Array of images.

It creates the video successfully, then I add audio to that created video file.

I create AVMuatableComposition object, I add Video and audio by creating AVAssetTracks and finally export into single video file with help of AVAssetsExportSession.

Suppose the first video without Audio is vdo.mp4 and final (After adding audio) is final.mp4, So my final.mp4 is lower in size and resolution than the vdo.mp4

Here is my code which combines the both file,

NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
NSError * error = nil;

AVMutableComposition * composition = [AVMutableComposition composition];

NSURL *url = [NSURL fileURLWithPath:filePath];

AVURLAsset * videoAsset = [AVURLAsset URLAssetWithURL:url options:nil];

AVAssetTrack * videoAssetTrack = [[videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];

AVMutableCompositionTrack *compositionVideoTrack = [composition addMutableTrackWithMediaType:AVMediaTypeVideo
                                                                            preferredTrackID: kCMPersistentTrackID_Invalid];

[compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero,videoAsset.duration) ofTrack:videoAssetTrack atTime:kCMTimeZero error:&error];

CMTime audioStartTime = kCMTimeZero;
for (   NSInteger i = 0;i< [mArrAudioFileNames count];i++ )
{
    NSString *audioFileName = nil;
    NSString *docsDir = nil;
    if ( [mArrAudioFileNames objectAtIndex:i] != [NSNull null]) {
        audioFileName = [mArrAudioFileNames objectAtIndex:i];
        docsDir = [[self dataFolderPathForAudio] stringByAppendingPathComponent:audioFileName];

    }else{
        //audioFileName = @" ";
        docsDir = [[NSBundle mainBundle] pathForResource:@"sample" ofType:@"mp3"];
    }

    // NSString *docsDir = [[self dataFolderPathForAudio] stringByAppendingPathComponent:audioFileName];
    AVURLAsset * urlAsset = [AVURLAsset URLAssetWithURL:[NSURL fileURLWithPath:docsDir] options:nil];

    AVAssetTrack * audioAssetTrack = [[urlAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
    AVMutableCompositionTrack *compositionAudioTrack = [composition addMutableTrackWithMediaType:AVMediaTypeAudio
                                                                                preferredTrackID: kCMPersistentTrackID_Invalid];

    [compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero,urlAsset.duration) ofTrack:audioAssetTrack atTime:audioStartTime error:&error];

    Float64 duration = CMTimeGetSeconds(urlAsset.duration);

    audioStartTime = CMTimeAdd(audioStartTime, CMTimeMake((int) ((duration * kRecordingFPS) + 0.5), kRecordingFPS));
}

AVAssetExportSession* assetExport = [[AVAssetExportSession alloc] initWithAsset:composition presetName:AVAssetExportPresetMediumQuality];
//assetExport.videoComposition = compositionVideoTrack;

 assetExport.outputFileType = AVFileTypeQuickTimeMovie;// @"com.apple.quicktime-movie";
assetExport.outputURL = [NSURL fileURLWithPath:outFilePath];

[assetExport exportAsynchronouslyWithCompletionHandler:
 ^(void ) {
     switch (assetExport.status)
     {
         case AVAssetExportSessionStatusCompleted:
             //                export complete
             NSLog(@"Export Complete");
             [self performSelectorOnMainThread:@selector(creatingVideoDone:)
                                    withObject:outFilePath waitUntilDone:NO];
             [assetExport release];
             break;
         case AVAssetExportSessionStatusFailed:
             NSLog(@"Export Failed");
             NSLog(@"ExportSessionError: %@", [assetExport.error localizedDescription]);
             // Set delegate to move to view
             if ( mDelegate!= nil && [mDelegate respondsToSelector:@selector(errorAlert:)])
             {
                 [self performSelectorOnMainThread:@selector(errorOccured:)
                                        withObject:[assetExport.error
                                                    localizedDescription]
                                     waitUntilDone:NO];
             }
             [assetExport release];
             break;
         case AVAssetExportSessionStatusCancelled:
             NSLog(@"Export Failed");
             NSLog(@"ExportSessionError: %@", [assetExport.error localizedDescription]);
             // Set delegate to move to view
             if ( mDelegate!= nil && [mDelegate respondsToSelector:@selector(errorAlert:)])
             {
                 [self performSelectorOnMainThread:@selector(errorOccured:)
                                        withObject:[assetExport.error
                                                    localizedDescription]
                                     waitUntilDone:NO];
             }
             [assetExport release];
             break;
     }
 }];

Any help is appreciated.

Thanks.