使用 RenderTargetBitmap 和 PngBitmapEncoder 时的 PNG 大小增长

我正在使用一种方法将一些磁贴图像合并为单个图像。但是如果我将其应用于四个 30kb- PNG 图像，生成的 PNG 图像将是 500K（~4 倍于我的预期）

这是我正在使用的代码的一部分（由Cédric Bignon建议）：

BitmapFrame frame1 = BitmapDecoder.Create(new Uri(path1), BitmapCreateOptions.None, BitmapCacheOption.OnLoad).Frames.First();
BitmapFrame frame2 = BitmapDecoder.Create(new Uri(path2), BitmapCreateOptions.None, BitmapCacheOption.OnLoad).Frames.First();
BitmapFrame frame3 = BitmapDecoder.Create(new Uri(path3), BitmapCreateOptions.None, BitmapCacheOption.OnLoad).Frames.First();
BitmapFrame frame4 = BitmapDecoder.Create(new Uri(path4), BitmapCreateOptions.None, BitmapCacheOption.OnLoad).Frames.First();
// Gets the size of the images (I assume each image has the same size)
int imageWidth = frame1.PixelWidth;
int imageHeight = frame1.PixelHeight;
// Draws the images into a DrawingVisual component
DrawingVisual drawingVisual = new DrawingVisual();
using (DrawingContext drawingContext = drawingVisual.RenderOpen())
{
    drawingContext.DrawImage(frame1, new Rect(0, 0, imageWidth, imageHeight));
    drawingContext.DrawImage(frame2, new Rect(imageWidth, 0, imageWidth, imageHeight));
    drawingContext.DrawImage(frame3, new Rect(0, imageHeight, imageWidth, imageHeight));
    drawingContext.DrawImage(frame4, new Rect(imageWidth, imageHeight, imageWidth, imageHeight));
}
// Converts the Visual (DrawingVisual) into a BitmapSource
RenderTargetBitmap bmp = new RenderTargetBitmap(imageWidth * 2, imageHeight * 2, 96, 96, PixelFormats.Pbgra32);
bmp.Render(drawingVisual);
// Creates a PngBitmapEncoder and adds the BitmapSource to the frames of the encoder
PngBitmapEncoder encoder = new PngBitmapEncoder();
encoder.Frames.Add(BitmapFrame.Create(bmp));
// Saves the image into a file using the encoder
using (Stream stream = File.OpenWrite(pathTileImage))
    encoder.Save(stream);

有人知道发生了什么吗？