Convert binary extracted from .getUserMedia () to .mp4

I am saving the user's video and audio using .getUserMedia () in javascript and sending it to a websocket listening in C # (MVC). I leave below the path that the data makes until it reaches the server.

In Javascript

var chunks = [];
var stream = null;
...

function getMedia() {
   var constraints = { audio: true, video: { width: 1280, height: 720 } };

   navigator.mediaDevices.getUserMedia(constraints)
   .then(function (mediaStream) {

    stream = mediaStream;

    if (hasWebSockets) {
        try {
            socket = new WebSocket(myC#SocketURL);

            ...
        }
    }

    mediaRecorder.ondataavailable = function (e) {

        chunks.push(e.data);
    }


    mediaRecorder.onstop = function (e) {

        if (socket.readyState == WebSocket.OPEN) {
            ReadAndSend(chunks[i]); // i aumenta em todos os ciclos, 
                                       indica a sequência dos dados. 
                                       esta sequência vai ser usada em C#
        }        
    }

function ReadAndSend(t) {
   var reader = new FileReader();

   reader.addEventListener("loadend", function () {

       var s = reader.result;
       var view = new Uint8Array(s);
       var binary = btoa(Uint8ToString(view));

       var toSend = {

           "sequencia" : i,
           "data": binary

       };


       var z = JSON.stringify(toSend);

       try {

           socket.send(z);
       }

}

function Uint8ToString(u8a) {
   console.log("Uint8ToString");
   var CHUNK_SZ = 0x64000;
   var c = [];
   for (var i = 0; i < u8a.length; i += CHUNK_SZ) {
       c.push(String.fromCharCode.apply(null, u8a.subarray(i, i + 
                                           CHUNK_SZ)));
   }
   return c.join("");

}

In CSharp

WebSocket webSocket = webSocketContext.WebSocket;

        try
        {

            var receiveBuffer = new ArraySegment<Byte>(new Byte[1024 * 1024 * 16]); ;

            while (webSocket.State == WebSocketState.Open)
            {

                WebSocketReceiveResult receiveResult = await webSocket.ReceiveAsync(receiveBuffer);


                ...
                byte[] payloadData = receiveBuffer.Array.Where(b => b != 0).ToArray();
                string receiveString = System.Text.Encoding.UTF8.GetString(payloadData, 0, payloadData.Length);

                JObject o = JObject.Parse(receiveString);
                TransFile transFileChunk = new TransFile();
                string ba = o["data"].ToString();
                transFileChunk.bytes = Convert.FromBase64String(ba);
                transmitedFiles[nomeDoFicheiro].Add(transFileChunk);



                 if (receiveResult.MessageType == WebSocketMessageType.Close)
                {
                    await webSocket.CloseAsync(WebSocketCloseStatus.NormalClosure, "", CancellationToken.None);

                    var files = transmitedFiles.Distinct().ToList();

                    foreach (KeyValuePair<string, List<TransFile>> ltf in files)
                    {

                        foreach (TransFile tf in ltf.Value.Where(m => m.bytes.Length != 0).OrderBy(m => m.sequence))
                        {
                            using (BinaryWriter writer = new BinaryWriter(File.Open("C://Path" + ltf.Key + ".mp4", FileMode.Append)))
                            {

                                writer.Write(tf.bytes);
                            }
                        }

                        transmitedFiles.Remove(ltf.Key); // vamos apagar o ficheiro do 'repositório'
                    }


                }

            }

Using FFMPEG to convert results, but the final file size is too large, which can not be because those videos are used by the user within the web app.