I came across this solution as a recommended result, so thought to share my solution here, as well. What you seem to be describing is equivalent to fitting an image inside its frame: image.contentMode = .scaleAspectFit Here's the CGAffineTransform equivalent for an AVAssetTrack: func scaleAspectFitTransform(for assetTrack: AVAssetTrack, into renderSize: CGSize) -> CGAffineTransform {   let naturalSize = assetTrack.naturalSize.applying(assetTrack.preferredTransform)   let absoluteSize = CGSize(width: abs(naturalSize.width),                height: abs(naturalSize.height))       let xFactor = renderSize.width / absoluteSize.width   let yFactor = renderSize.height / absoluteSize.height       let scaleFactor = min(xFactor, yFactor)       var output = assetTrack.preferredTransform.scaledBy(x: scaleFactor, y: scaleFactor)   output.tx = assetTrack.preferredTransform.tx * scaleFactor   output.ty = assetTrack.preferredTransform.ty * scaleFactor       let compressedSize = CGSize(width: absoluteSize.width * scaleFactor,                 height: absoluteSize.height * scaleFactor)       let widthDiff = renderSize.width - compressedSize.width   let heightDiff = renderSize.height - compressedSize.height   let plainTranslation = CGAffineTransform.identity.translatedBy(x: widthDiff / 2,                                   y: heightDiff / 2)       return output.concatenating(plainTranslation) Then, apply it to your layer instructions: guard let sourceVideoTrack = sourceAVAsset.tracks(withMediaType: .video).first else { return } let transform = scaleAspectFitTransform(for: sourceVideoTrack, into: renderSize) var layerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: outputVideoTrack) layerInstruction.setTransform(transform, at: .zero) let compositionInstruction = AVMutableVideoCompositionInstruction() compositionInstruction.timeRange = CMTimeRangeMake(start: .zero, duration: videoAsset.duration) compositionInstruction.layerInstructions = [layerInstruction]     let videoComposition = AVMutableVideoComposition() videoComposition.renderSize = renderSize videoComposition.instructions = [compositionInstruction]

Solution without a large switch/case: https://developer.apple.com/forums/thread/705269?answerId=741570022#741570022