Why doing POST throws an exception in MVC 4?

I am trying to do a POST like this:

    HttpClient hc = new HttpClient();
    byte[] bytes = ReadFile(@"my_path");

    var postData = new List<KeyValuePair<string, string>>();
    postData.Add(new KeyValuePair<string, string>("FileName", "001.jpeg"));
    postData.Add(new KeyValuePair<string, string>("ConvertToExtension", ".pdf"));
    postData.Add(new KeyValuePair<string, string>("Content", Convert.ToBase64String(bytes)));

    HttpContent content = new FormUrlEncodedContent(postData);
    hc.PostAsync("url", content).ContinueWith((postTask) => {
    postTask.Result.EnsureSuccessStatusCode();
    });

but I receive this exception:

Invalid URI: The Uri string is too long.

complaining about this line: HttpContent content = new FormUrlEncodedContent(postData);. For small files it works but I don't understand why for larger ones it doesn't?

When I do POST the content can be larger...Then why it complains about URI?