When a QWebSocket sends a large binary message (I used 1352101 bytes), the underlying message is understood to be broken into frames.

webSocket->sendBinaryMessage(data); // returns 1352101 bytes as expected.

To confirm on the server side, the receiving QWebSocketServer receives the following binary frames from the incoming socket:
524288 (isLastFrame = false)
524288 (isLastFrame = false)

• the last frame is never received and the QWebSocket closes internally with close code 1000 (Normal). The binaryMessageReceived signal is never emitted from the socket.

The specification of websockets has a much higher upper limit than a couple of Mb so I don't understand why the message hasn't been received in its entirety.