WebRequest fails when using params over a certain size (Twitter Streaming API)

This may not be Twitter specific, but I am calling an API and when my params pushes over a certain size limit, the request fails with either a webException (sendFailure or receiveFailure).

I've pinpointed the max length of the parameters part as being between 12251 and 12269 characters long. Below 12251, it always works fine - above 12269 it fails!

In all machine.config files on the machine, I have this:

    <httpRuntime maxRequestLength="32768"/>

My oAuth implementation is absolutely fine as well - as below this length, the parameters part works fine.

For info, here is the C# part (but this is not needed because as I've said, the code works it's just when this request length hits a ceiling):

    private void InitialiseWebStreamRequest()
    {
        var userAgent = <<userAgent>>;

        var requestBuilder = new Twitterizer2.WebRequestBuilder(new Uri("https://stream.twitter.com/1.1/statuses/filter.json"), Twitterizer2.HTTPVerb.POST, <<tokens>>, userAgent);

        var parameters = string.Join(",", arrayOfNumbers);

        requestBuilder.Parameters.Add("follow", parameters);

        WebRequest = requestBuilder.PrepareRequest();            
        WebRequest.ContentType = "application/x-www-form-urlencoded";
        WebRequest.Timeout = 120000;
    }

The web "PrepareRequest" part is straight from the open source Twitterizer2 library and this prepares a request with oAuth as follows:

public HttpWebRequest PrepareRequest()
    {
        SetupOAuth();

        formData = null;
        string contentType = string.Empty;

        if (!Multipart)
        {   //We don't add the parameters to the query if we are multipart-ing
            AddQueryStringParametersToUri();
        }
        else
        {
            string dataBoundary = "--------------------r4nd0m";
            contentType = "multipart/form-data; boundary=" + dataBoundary;

            formData = GetMultipartFormData(Parameters, dataBoundary);

            this.Verb = HTTPVerb.POST;
        }

        HttpWebRequest request;
        request = (HttpWebRequest)WebRequest.Create(this.RequestUri);

        if (this.UseCompression == true)
            request.AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Deflate;
        else
            request.AutomaticDecompression = DecompressionMethods.None;

        if (this.Proxy != null)
            request.Proxy = Proxy;

        if (!this.UseOAuth && this.networkCredentials != null)
        {
            request.Credentials = this.networkCredentials;
            request.UseDefaultCredentials = false;
        }
        else
        {
            request.UseDefaultCredentials = true;
        }

        request.Method = this.Verb.ToString();

        request.ContentLength = Multipart ? ((formData != null) ? formData.Length: 0) : 0;


        if (this.UseOAuth)
        {
            request.Headers["Authorization"] = GenerateAuthorizationHeader();
        }

        if (Multipart)
        {   //Parameters are not added to the query string, post them in the request body instead
            request.ContentType = contentType;

            IAsyncResult getRequestStreamResult = request.BeginGetRequestStream(
                res =>
                {
                }, null);

            using (Stream requestStream = request.EndGetRequestStream(getRequestStreamResult))
            {
                if (formData != null)
                {
                    requestStream.Write(formData, 0, formData.Length);
                }
            }
        }

        return request;
    }

Anyone have any ideas?