filesize not update after change file

PHP code

echo filesize("a.jpg")."<br/>";

imagejpeg(imagecreatefromjpeg("a.jpg"), "a.jpg", 50);

echo filesize("a.jpg");

Output

73104
73104

I have change quality of image but filesize not changed!!!

However, the file size changed?!

why function filesize not updated immediate after change file?

My image(size 72k)