how to serialize ArrayBuffer into BSON object for websocket transfer in javascript

Question

I want encode a javascript object with a ArrayBuffer element into BSON and transfer it over websocket. However the serialized BSON object is 10 time bigger than it should be.

The code is something like this:

var A = {buffer: new ArrayBuffer(1024)};

bson_buffer_size = BSON.calculateObjectSize(A) // returns 9164, I am expecting ~1024

My understanding of BSON serializer must be wrong. Does anyone know how I can create a compact BSON object with a binary buffer field?