Combining two m4a files

[cncool]

I can successfully combine two m4a files one after the other using AVMutableComposition and AVExportSession

    AVMutableComposition* composition = [AVMutableComposition composition];
    AVURLAsset* audioAsset1 = [[AVURLAsset alloc]initWithURL:[NSURL fileURLWithPath:audioFile1Path] options:nil];
    AVURLAsset* audioAsset2 = [[AVURLAsset alloc]initWithURL:[NSURL fileURLWithPath:audioFile2Path] options:nil];
    
    AVMutableCompositionTrack *audioTrack1 = [composition addMutableTrackWithMediaType:AVMediaTypeAudio 
                                                                      preferredTrackID:kCMPersistentTrackID_Invalid];
    
    [audioTrack1 insertTimeRange:CMTimeRangeMake(kCMTimeZero, audioAsset1.duration)
                         ofTrack:[[audioAsset1 tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0]
                          atTime:kCMTimeZero
                           error:nil];
    [audioTrack1 insertTimeRange:CMTimeRangeMake(kCMTimeZero, audioAsset2.duration) 
                         ofTrack:[[audioAsset2 tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0] 
                          atTime:audioAsset1.duration
                           error:nil];
    
    AVAssetExportSession * ex = [[AVAssetExportSession alloc] initWithAsset:[audioTrack1 asset] presetName:AVAssetExportPresetAppleM4A];

When I export, the resulting file is like 10x bigger than the size of the sum of the other two files. How would I combine the files but not have the file size be so big?