Why "readFile" method places file's content into the memory in contrast with "readFileSync"?

Here is an example of NodeJS code (synchronous version):

var fs = require('fs');
var path = './parseLogFiles/reports';
var counter = 0;
var totalFileSize = 0;
var fileName;
var fullPath;

function toMb (byteVal) {
    return (byteVal / 1048576).toFixed(2);
}

var filesList = fs.readdirSync(path);

console.log('Memory usage before files read:', toMb(process.memoryUsage()['heapUsed']) + ' MB');

for (var i = 0, len = filesList.length; i < len; i++) {

    fileName = filesList[i];

    if (fileName) {

        fullPath = path + '/' + fileName;

        totalFileSize += fs.statSync(fullPath)['size'];

        try {
            fs.readFileSync(fullPath, {encoding: 'utf8'});
        } catch(err){
            console.log('err: ', err);
        }

    }

}

console.log('Memory usage after files read:', toMb(process.memoryUsage()['heapUsed']) + ' MB');
console.log('Total files size:', toMb(totalFileSize) + ' MB');

I have got the following results:

Memory usage before files read: 22.45 MB

Memory usage after files read: 23.31 MB

Total files size: 258.19 MB

Here is asynchronous version:

...

for (var i = 0, len = filesList.length; i < len; i++) {

    fileName = filesList[i];

    if (fileName) {

        fullPath = path + '/' + fileName;

        (function(fullPath){
            fs.stat(fullPath, function(err, stat){
                totalFileSize += stat['size'];
                fs.readFile(fullPath, {encoding: 'utf8'}, function(){
                    if (++counter === len) {
                        console.log('Memory usage after files read:', toMb(process.memoryUsage()['heapUsed']) + ' MB');
                        console.log('Total files size:', toMb(totalFileSize) + ' MB');
                    }
                });
            });
        })(fullPath);

    }

}

I have got the following results:

Memory usage before files read: 22.45 MB

Memory usage after files read: 437.88 MB

Total files size: 258.19 MB

Why does it happen (23.31 MB vs 437.88 MB)?