I came across this issue also. There are a couple of ways to fix it.
Either you can alter the per protocol receive buffer size, which can be set in the WebSocket::init method, where the _wsProtocols member is setup.
ie. After line 326 in WebSocket.cpp:
_wsProtocols[i].rx_buffer_size = 1024 * 64; // 64;
I’ve done the second option and have attached my patched WebSocket.cpp and .h files in case anyone is interested. They are patched from the 2.1.4 release version.
Hope that helps,