Differing Blob sizes before and after reading with FileReader

Question

I am trying to read a Blob withe Javascript FileReader API using the following code:

reader = new FileReader();
reader.onloadend = function(evt) {
  console.log(new Blob([reader.result]));
}
// outBlob defined elsewhere
console.log(outBlob);
reader.readAsBinaryString(outBlob);

Running this in Chrome gives:

          Blob {type: "undefined", size: 70012, slice: function}
          Blob {type: "", size: 105244, slice: function}

My question is, why are the sizes different here?