Why does this load the whole ubuntu iso in memory?

stream.js

var fs = require('fs');

var path = "/Users/mbc/Downloads/ubuntu-10.04.1-server-amd64.iso";

var stream = fs.createReadStream(path);
var out = fs.createWriteStream('tmp/testing.iso');

stream.on('data', function(data) {
  console.info("Streaming data "+data.length);
  out.write(data);
});

stream.on('end', function() {
  console.info("Streaming done");
  out.end();
});