It seems libsoup doesn't handle binary files like images and such. I asked on the libsoup mailing list and was suggested it was a Vala implementation problem (http://markmail.org/message/miwspdv5oknzq3ct)

This example program given an image of 4 bytes when it's actually 444 k.

===============================================================================

//! download-binary.vala
//!
//! compile: 
//! valac -o test --pkg libsoup-2.4 --pkg gio-2.0 --thread download-binary.vala

int main(string[] args)
{
  var url = "http://www.poppa.se/blog/data/images/gnome-shield-4.jpg";
  var sess = new Soup.SessionSync();
  var mess = new Soup.Message("GET", url);

  sess.add_feature = new Soup.Logger(Soup.LoggerLogLevel.HEADERS, -1);
  sess.send_message(mess);

  file_put_contents(Path.get_basename(url), mess.response_body.data);

  return 0;
}

void file_put_contents(string file, string contents)
{
  try {
    var f = File.new_for_path(file);

    if (f.query_exists(null))
      f.delete(null);

    var fs = f.create(FileCreateFlags.NONE, null);
    if (!f.query_exists(null)) {
      warning("Unable to create file \"%s\"!", file);
      return;
    }
    var ds = new DataOutputStream(fs);
    ds.put_string(contents, null);
  }
  catch (Error e) {
    warning("Error writing file: %s", e.message);
  }
}

===============================================================================

I apologize if this actually is not a bug but rather a mis-usage from my side.