Reducing the size of a video exported with AVAssetExportSession - iOS Swift

I'm currently exporting a video in the following way:

   let exporter = AVAssetExportSession.init(asset: mixComposition, presetName: AVAssetExportPreset1280x720)
   exporter?.outputURL = outputPath
   exporter?.outputFileType = AVFileType.mp4
   exporter?.shouldOptimizeForNetworkUse = true
   exporter?.videoComposition = mainCompositionInst

A 15s video consumes about 20MB in data. Comparing this to Snapchat's 2MB videos, this number seems totally unacceptable.

I already reduced the quality of the export- and capture session (1280x720).

The video is filmed on a custom camera. UIImagePickerController is not used.

AVAssetExportSession is used with default settings.

Is there any way I can reduce the size of my videos? Thanks a lot!

EDIT 1: I tried to use this library: https://cocoapods.org/pods/NextLevelSessionExporter

Unfortunately, this creates sizing problems and removes my audio:

// Creating exporter
    let exporter = NextLevelSessionExporter(withAsset: mixComposition)
    exporter.outputURL = outputPath
    exporter.outputFileType = AVFileType.mp4
    exporter.videoComposition = mainCompositionInst

    let compressionDict: [String: Any] = [
        AVVideoAverageBitRateKey: NSNumber(integerLiteral: 2500000),
        AVVideoProfileLevelKey: AVVideoProfileLevelH264BaselineAutoLevel as String,
        ]

        exporter.videoOutputConfiguration = [
            AVVideoCodecKey: AVVideoCodecType.h264,
            AVVideoWidthKey: NSNumber(integerLiteral: 1280),
            AVVideoHeightKey: NSNumber(integerLiteral: 720),
            AVVideoScalingModeKey: AVVideoScalingModeResizeAspectFill,
            AVVideoCompressionPropertiesKey: compressionDict
        ]

        exporter.audioOutputConfiguration = [
            AVFormatIDKey: kAudioFormatMPEG4AAC,
            AVEncoderBitRateKey: NSNumber(integerLiteral: 128000),
            AVNumberOfChannelsKey: NSNumber(integerLiteral: 2),
            AVSampleRateKey: NSNumber(value: Float(44100))
        ]

To reduce file size try these properties for setting up HEVC codec (use cocoa pod NextLevelSessionExporter):

let compressionDict: [String: Any] = [
AVVideoAverageBitRateKey: NSNumber(integerLiteral: 2500000), //lower it if you wish
AVVideoProfileLevelKey: AVVideoProfileLevelH264BaselineAutoLevel as String,
]
exporter.videoOutputConfiguration = [
    AVVideoCodecKey : AVVideoCodecType.hevc,
    AVVideoWidthKey : NSNumber(integerLiteral: 1280),
    AVVideoHeightKey: NSNumber(integerLiteral: 720),
    AVVideoScalingModeKey: AVVideoScalingModeResizeAspectFill,
    AVVideoCompressionPropertiesKey: compressionDict
]

You need to upgrade to macOS High Sierra and iOS 11 in order to use HEVC video codec. But if you can't use HEVC for some reason, use regular H.264 with lower bitrate.

AVVideoCodecKey : AVVideoCodecType.h264:

Also, look at this SO post about video bitrate in iOS.