降低图像分辨率增加文件大小

我有一个MVC应用程序，你可以上传一张图片，它会被调整到最大。50 kb。我在while循环中调整大小，但问题是当我减少图片的宽度和高度时，文件大小会增加。在某一点上，尺寸变小了，但以质量为代价

    Request.InputStream.Position = 0;
    string Data = new System.IO.StreamReader(Request.InputStream).ReadToEnd();
    var Base64 = Data.Split(',')[1];
    var BitmapBytes = Convert.FromBase64String(Base64);
    var Bmp = new Bitmap(new MemoryStream(BitmapBytes));
    while (BitmapBytes.Length > 51200)
    {
        int Schritte = 20; //I tested here also with 300
        int maxWidth = Bmp.Width;
        maxWidth = maxWidth - Schritte;
        int maxHeight = Bmp.Height;
        maxHeight = maxHeight - Schritte;
        Bmp = ScaleImage(Bmp, maxWidth, maxHeight);
        var base64 = ReturnImageAsBase64(Bmp);
        BitmapBytes = Convert.FromBase64String(base64);
    }

调整大小的代码:

public static Bitmap ScaleImage(Image image, int maxWidth, int maxHeight)
{
    var ratioX = (double)maxWidth / image.Width;
    var ratioY = (double)maxHeight / image.Height;
    var ratio = Math.Min(ratioX, ratioY);
    var newWidth = (int)(image.Width * ratio);
    var newHeight = (int)(image.Height * ratio);
    Bitmap newImage = new Bitmap(newWidth, newHeight);
    using (Graphics gr = Graphics.FromImage(newImage))
    {
        gr.SmoothingMode = SmoothingMode.HighQuality;
        gr.InterpolationMode = InterpolationMode.HighQualityBicubic;
        gr.PixelOffsetMode = PixelOffsetMode.HighQuality;
        gr.DrawImage(image, new Rectangle(0, 0, newWidth, newHeight));
     }
     return newImage;
}

我以66964字节的大小开始。在循环的第一个回合后，它是85151字节，尽管宽度减少了300像素，高度减少了420像素。