调整大小后如何缩小图像文件大小

Question

我使用下面编写的代码来缩小图像大小并保持宽高比。我调整大小的图像是1920 * 1275 1.9mbytes，原始图像大小是4288×2848 1.8mbytes。图像大小已调整大小但文件大小未调整。我如何减小文件大小？

目前，我想保持图像的质量。

var sourceImage = new UIImage(source); 
        var sourceSize = sourceImage.Size; 
        var maxResizeFactor = Math.Max(targetWidth / sourceSize.Width, targetHeight / sourceSize.Height);
        if (maxResizeFactor > 1) return source;

        var ratioImage = sourceSize.Width / sourceSize.Height;
        var ratioScreen = targetWidth / targetHeight;
        var newWidth = 0.0f;
        var newHeight = 0.0f;

        if (ratioScreen > ratioImage)
        {
            newWidth = (float)(sourceSize.Width * (targetHeight / sourceSize.Height));
            newHeight = targetHeight;
        }
        else 
        {
            newWidth = targetWidth;
            newHeight = (float)(sourceSize.Height * (targetWidth / sourceSize.Width));
        }

        UIGraphics.BeginImageContext(new SizeF((float) newWidth, (float) newHeight));
        sourceImage.Draw(new RectangleF(0, 0, (float)newWidth, (float)newHeight));
        var resultImage = UIGraphics.GetImageFromCurrentImageContext();
        UIGraphics.EndImageContext();

        NSError error;
        if (MimeType.IsPNG(source))
            resultImage.AsPNG().Save(destination, true, out error);
        else
            resultImage.AsJPEG().Save(destination, true, out error);

        if (error != null)
            return source;
        else
            return destination;