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.

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

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