I cannot reproduce the problem with your image. Can you try saving the image before you 'return' it? Maybe you are doing something in between that is causing this behavior?
The Magick.NET library directly calls the ImageMagick methods. It does not wrap the command line. The image will only be compressed when you call image.Write() or save the Bitmap you created with ToBitmap().
using (MagickImage image = new MagickImage(pathToPhotoInString)) { image.Resize(2100, 1400); Bitmap bitmap = image.ToBitmap(); bitmap.Save(@"C:\test.jpg"): return bitmap; }