HarmonyOS 鸿蒙Next 使用OffscreenCanvas绘制图片转成ArrayBuffer后获取到的字节大小与原图相比差距过大

HarmonyOS 鸿蒙Next 使用OffscreenCanvas绘制图片转成ArrayBuffer后获取到的字节大小与原图相比差距过大
在上传图片时，使用OffscreenCanvas绘制图片转成ArrayBuffer,获取到的字节数与通过stat获取到的差异过大，比原图的字节数大了近3倍，这个是什么原因呀？

async function imageAddWatermark(imageInfo: PicListItem, waterConfig: UploadImgParams, iSize: number):Promise<image.PixelMap> {

  const imgW = px2vp(imageInfo.width)

  const imgH = px2vp(imageInfo.height)

  // 这个回调的pixelMap就是当前图片的pixelMap

  let offScreenCanvas = new OffscreenCanvas(imgW, imgH)

  console.log('finally iSize 原图字节大小 ==', iSize)

  let OffScreenContext = offScreenCanvas.getContext("2d")

  // OffScreenContext.imageSmoothingEnabled = false;

  OffScreenContext.drawImage(imageInfo.pix, 0, 0)

  // 获取新的 PixelMap

  let newPixelMap = OffScreenContext.getPixelMap(0, 0, offScreenCanvas.width, offScreenCanvas.height)

  let imgArrayBuffer = await imagePacker(newPixelMap)

  const sPixSize = imgArrayBuffer.byteLength

  console.log('finally sPixSize canvas画出来的图片字节大小 ==', sPixSize)

  return newPixelMap

}