调整图像大小，但仍然有很多字节(UWP)

我有这段代码来调整存储文件的图像大小:

var file = await ImageChooser.GetSelectedImageAsStorageFile();
using (var sourceStream = await file.OpenAsync(FileAccessMode.Read))
{
    int maxWidth = 500, maxHeight = 500;
    BitmapImage sourceImage = new BitmapImage();
    sourceImage.SetSource(sourceStream);
    var origHeight = sourceImage.PixelHeight;
    var origWidth = sourceImage.PixelWidth;
    var ratioX = maxWidth / (float)origWidth;
    var ratioY = maxHeight / (float)origHeight;
    var ratio = Math.Min(ratioX, ratioY);
    var newHeight = (uint)(origHeight * ratio);
    var newWidth = (uint)(origWidth * ratio);
    BitmapDecoder decoder = await BitmapDecoder.CreateAsync(sourceStream);
    BitmapTransform transform = new BitmapTransform() { ScaledHeight = newHeight, ScaledWidth = newWidth };
    PixelDataProvider pixelData = await decoder.GetPixelDataAsync(
    BitmapPixelFormat.Rgba8,
    BitmapAlphaMode.Straight,
    transform,
    ExifOrientationMode.RespectExifOrientation,
    ColorManagementMode.DoNotColorManage);
    using (var destinationStream = await file.OpenAsync(FileAccessMode.ReadWrite))
    {
        var propertySet = new BitmapPropertySet();
        var qualityValue = new BitmapTypedValue(0.3, PropertyType.Single);
        propertySet.Add("ImageQuality", qualityValue);
        BitmapEncoder encoder = await BitmapEncoder.CreateAsync(BitmapEncoder.JpegEncoderId, destinationStream, propertySet);
        encoder.SetPixelData(BitmapPixelFormat.Rgba8, BitmapAlphaMode.Premultiplied, newWidth, newHeight, 96, 96, pixelData.DetachPixelData());
        await encoder.FlushAsync();
    }
}

问题是，即使调整大小也会随着输入字节的数量而继续，例如，如果一个图像是4MB，它会继续将输出字节的大小调整为4MB。