Parallel execution in Node.js.

Hi All, I was going through the chapter 8, callbacks. In one of the asynchronous flow example

const { readFile } = require('fs')
const [ bigFile, mediumFile, smallFile ] = Array.from(Array(3)).fill(__filename)

const print = (err, contents) => {
  if (err) {
    console.error(err)
    return
  }
  console.log(contents.toString())
}
readFile(bigFile, print)
readFile(mediumFile, print)
readFile(smallFile, print)

it is mentioned that if these were of different file sizes the order of execution will be
smallFile, mediumFile and BigFile. However i tried running this example by assigning BigFile-> package.json MediumFile->index.js and smallFile->format.js from the my-package example.
The order of execution was bigFile, MediumFile and smallFile. So my question is how it is asynchronous? it seems that it is executing in the sequential manner. Can you explain a bit more on this or maybe share an example how smallFile will be executed first.
Thank you.